Questions and Answers to Citrus Management

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Questions and Answers to
Citrus Management
Third Edition
Peggy A. Mauk, Ph.D.
Tom Shea

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Page No.
Varieties, planting and tree care -
Fertilization and irrigation practices -
Diseases, pests, insects and other problems -
References -
Index -
To simplify our information, it is sometimes necessary to use trade names of products or equipment. No
endorsement of named products is intended nor is criticism implied of similar products that are not

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Questions and Answers to
Citrus Management
Peggy A. Mauk, Ph.D.
Subtropical Horticulture Farm Advisor
Tom Shea
University of California Cooperative Extension
21150 Box Springs Road, Suite 202, Moreno Valley, CA 92557-8718
Revised from: F. F. Laemmlen, and G. W. Witney. 1994. Some answers to common questions asked by backyard citrus
growers. In house publication, UCCE Riverside County.

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1. Which citrus varieties do best in Southern California?
A. Grapefruit - Depending on the variety, fruit mature starting as early as November. Depending on the
location, some fruit can hold on the tree until September.
1. Marsh Seedless is white fleshed, juicy with fine flavor. High summer temperatures produce best
2. Ruby Red is light pink to red in warm summer climates. Juicy with fine flavor.
3. Rio Red has a deep red internal color, excellent flavor and is juicy. Needs summer heat. Does well
in lower desert valleys.
4. Star Ruby has the deepest colored of the red grapefruit with a less acidic fine flavor. Needs some
heat to develop the best flavor but it is not suitable for the desert. This variety is characterized by
white to yellow flecks in the leaves. It ripens in early summer.
5. Flame is red fleshed and has a red blush on the skin like the other red/pink grapefruit. It ripens in
late summer in the Riverside area.
6. Oroblanco, a UCR bred grapefruit-pummelo cross, is very sweet, juicy and low in acid. It ripens in
January and holds well into early summer.
7. Melogold, a UCR bred sibling to Oroblanco, is white fleshed, seedless like Oroblanco but juicier.
Ripens similar to Oroblanco.
8. Cocktail, a pummelo-mandarin hybrid developed by UCR, very sweet and juicy. Great for juice.
Ripens in late-December in Riverside.
B. Lemons
1. Lisbon lemon: Has some cold resistance, very heat tolerant, vigorous and thorny tree. Highly
productive, high quality fruit. Fruit mature mostly in fall to winter. On the coast, trees can bear some
fruit year round.
2. Eureka lemon: Cold sensitive, nearly thornless, very attractive tree, productive - high quality fruit.
Tree bears year round on the coast, fall and winter in the low desert valleys, and winter to spring
production in the inland Riverside areas.
a. Variegated Pink- a sport or mutation of Eureka that has variegated (green-and-white striped)
leaves and immature fruit striped green and cream, mature flesh is light pink plus the tree
itself is smaller making it very garden-friendly.
3. Improved Meyer lemon: Although it is considered to be a lemon, it is probably a lemon-sweet orange
hybrid. Thus it is fairly cold resistant, similar to sweet orange. Fruit are round, thin skinned, and
almost orange in color. Very juicy, with less acid than other lemons. Bears year round.
C. Limes
1. Bearss Lime (Tahitian-type Lime): Seedless fruit, much larger and milder flavor than the Mexican
lime. Not as cold hearty as a lemon. Of the limes, this is the most suitable to Southern California.
2. Mexican Lime (Key Lime): Very frost sensitive and is only suited to more tropical areas that do not
receive any frost (coastal areas). Thornless Mexican Lime is also available and is equally frost
3. Limequat (lime X kumquat hybrid): Lime-like flavor that can substitute for a lime. This tree is more
frost tolerant and can be planted in areas that receive an occasional frost.

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D. Oranges
1. Navels:
a. Washington: Large seedless fruit, most commonly eaten fresh (not juiced). Suited to cooler
production areas, does not produce high quality fruit in the desert. Produces well in San Diego
County, Orange County, Riverside, San Bernardino, and Redlands areas. Fruit splitting in the fall
and winter is a common problem and often related to irrigation practices weather conditions (see
Q. 38). Harvested from January through April in home gardens. This navel is the standard by
which all other navels are judged.
b. Cara Cara: this navel orange has reddish pink flesh. The pink color is similar to that of the red
grapefruits, however, it is similar to the Washington navel in taste and harvest time (February
through March).
c. Lane Late: ripens late in the season, extending the harvest of navels into early summer.
2. Valencia orange: Often called “juice” oranges. Thin skinned, smaller fruit with very juicy pulp. Tends
to alternate bear (see Q. 10). Ripens later than Navel (early summer through fall) - fruit store well on
the tree but may re-green in the summer. Seedless variety is Delta.
3. Blood oranges: Moro (better color) and Tarocco (better flavor) do well in inland and coastal areas.
Almost seedless fruit with a deep red coloration. Flavor is berry-like. Attractive spreading tree.
E. Mandarins (Tangerines) and Tangelos
1. Satsuma Mandarin
Satsuma mandarins produce easy-to-peel and seedless fruit. The varieties, Dobashi beni, Okitsu wase
and Owari all thrive in cooler parts of Southern California. Satsuma is sensitive to high temperature
and thus there are no Satsuma varieties suitable to plant in the lower desert valleys. Satsuma’s are the
most cold hardy citrus trees of commercial importance. They are also the earliest fruit to ripen. Fruit
from both Dobashi beni and Okitsu wase mature at the end of October. Owari ripens a month later. If
fruit are left on the tree they rapidly becomes puffy and insipid, however, fruit store well off of the
tree. Some Owari strains have degenerated into poor trees due to its ability to sport readily producing
new strains that are not productive. Most Satsuma varieties tend to alternate bear (see Q. 10).
2. Other Mandarins
a. Dancy, the Holiday tangerine, is rich, juicy with seeds, and often, sold with leaves attached.
b. Seedless Kishu has small fruit slightly larger than a golf ball, mild, sweet, truly seedless, quite
juicy and extremely easy to peel. The fruit matures in November and holds until January.
c. Gold Nugget is a seedless variety, developed and released by UCR that has a sweet, rich flavor
with a somewhat bumpy skin that peels easily. Ready in March and holds well through August.
d. Pixie, also developed by UCR, has a sweet, mild flavor, without seeds, holds well and peels
easily. In the inland area, fruit can be harvested as early as mid-February and go through early
e. Clementine (Algerian) has bright orange, juicy flesh with sweet, very rich flavor. Bears from
December through February. Must have Dancy pollinator for good fruit production.
f. Fairchild is very juicy with richly sweet flesh and seeds. Needs Temple (Royal) pollinator.
g. Shasta Gold™ (TDE #2) was recently developed and released by UCR. The flesh is seedless,
bright orange and juicy. The flavor is richly sweet. In the inland are, the season is mid-February
to mid-March and holds well on the tree into April or May.

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h. Tahoe Gold™ (TDE #3) another new release by UCR. The flesh of the fruit is seedless, bright
orange, finely-textured and juicy. The flavor is rich and sweet. Its season is mid-January to mid-
i. Yosemite Gold™ (TDE #4) also from UCR’s citrus breeding program. The flesh is seedless,
bright orange, finely-textured and juicy. The flavor is richly sweet. The season is January to mid-
March and holds well on the tree into April.
3. Tangelos (a cross between grapefruit and mandarins)
a. Orlando tangelo: Fruit ripen in November-January. Fruit peels poorly and it contains many seeds.
May need pollinator for more fruit production. (See Q. 7).
b. Minneola tangelo: Large, red-orange fruit; peels well; few seeds; rich tart flavor. Fruit ripen in
December-February. May need pollinator for better fruit production. (See Q. 7).
F. Kumquats
Meiwa and Nagami
These are the most cold-hardy of all citrus types. Tree foliage can withstand temperatures below 20º F
and therefore can be grown in areas that are too cold for most citrus. Fruit, however, are more cold-
sensitive. Trees are small, less than 12 ft tall. Fruit are eaten whole, peel and all. Meiwa have round fruit
with a sweet rind and flesh, excellent for eating. Nagami have oval-shaped fruit with a sweet rind and tart
flesh. Nagami is the most common variety found in grocery stores. Nordman Seedless Nagami, a new
release, has really nice fruit 1 to 1½ inches long without seeds, therefore, especially easy to eat or
2. When should I plant, how deep should I plant and how big a tree should I plant?
The best planting time is after frost danger (after February 15 in the Riverside area) and before the onset
of hot weather. Although fall planting can be successful it is generally better to wait until spring. Always
choose a location that has full sun throughout the day.
It is best to plant in well-drained soil (see Q. 3). Dig the planting hole as deep as the root ball and as wide
or wider than the root ball. Be certain that the tree is not buried below the graft union. The graft union
(slight dogleg shape in the trunk) should be located several inches above the soil level. Trees buried too
deep may not survive. It is always best to fill the planting hole with the same soil that was removed when
digging the hole. Do not add any mulch or potting soil into the tree hole. These materials retain more
water and may increase the chances of root rot.
The best size of tree to plant is a tree with a trunk that is approximately ½ inch in diameter. Trees in pots
larger than 5 gallons may not grow as vigorously as trees in 5 gallon or smaller pots. In general, trees that
are container grown for long periods may have compromised roots systems. Additionally, there is a
substantial savings in purchasing small trees versus large container grown trees. Commercial growers
plant trees with less than a 1.5 inch diameter trunk. (See also Q. 5).
3. What soil is best for citrus?
Well-drained loam or sandy loam soils is best for citrus. High salinity, heavy clay and/or poorly drained
soils are detrimental to citrus.
In western Riverside County, many of the soil types have a limited amount of topsoil and often times
there is a hard pan below the topsoil. Desert soils generally have several clay lenses (layers) under the
top sandy soil. In order to ensure proper drainage and good root penetration, homeowners should loosen
compacted soils and mix the soil layers. For desert soils, dig a 6 ft. X 6 ft. X 6 ft. pit and refill with the
same soil. This will minimize the clay layers under the sandy top soil.

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4. When should I prune a citrus tree?
In general, citrus trees do not require frequent or severe pruning. Removal of deadwood and very weak
shoots or suckers is the only pruning required. Avoid pruning grapefruit because of the potential
development of Rio Grande gummosis disease (see Q. 34). In general, no pruning is required for the first
15-20 years except for the removal of deadwood. Lemons are an exception and they require thinning and
heading back to keep control of vigorous shoot growth. Light pruning can be done any time, heavy
pruning should be done during the winter months so that tree limbs are not sunburned (December-
January). Manage the tree’s size so that it easy for you to harvest.
5. Should I protect the trunk and large limbs from sunburn?
Yes, particularly lemon trees and newly planted trees. Wrap the trunk of the newly planted tree with
newspapers or tree wraps and tie loosely. The trunk and exposed branches can be painted with a white
wash; white flat water-based non-enamel paint (diluted 1:1 with water). In the lower desert, you must
control the ants that feed on the tree trunk under the tree wrap. If left uncontrolled, the ants can girdle the
new tree and kill it.
6. Do citrus trees need cross-pollination?
Some tangelos and mandarins need cross-pollination. For example, Minneola tangelo, Orlando tangelo,
Clementine mandarin and Fairchild mandarin need a cross pollinator for best fruit production (see Q. 7).
7. What is a good pollinator for Minneola, Orlando and Fairchild?
• Minneola: Temple, Dancy, Kinnow, or Clementine
• Orlando: Temple, Dancy, or Kinnow
• Fairchild: Orlando, Clementine, Kinnow, or Temple (Royal)
8. My Temple tangors are dying. Why?
There may be a number of causes. Temple tangors (a mandarin-orange cross) are sensitive to frost. A
freeze may “set back” the tree for years. Poorly drained, heavy soils can kill Temples (also see Q. 30).
Over-production for two or more consecutive years may also cause dieback or tree death. To reduce the
stress of over-production, thin excess fruit early (before they reach the size of marbles).
9. Should fruit be left on the tree after maturing?
No. Leaving fruit on the tree after maturing may contribute to a smaller crop and perhaps, more fruit drop
for the next crop. Most citrus will store on the tree for a short time, however, pick all mature fruit before
the weather turns hot. See also Q. 10
10. Why does my tree produce heavily one year and almost nothing the next?
Certain citrus types such as Valencia oranges or some mandarins have a tendency to have a year with
heavy fruit production followed by a year with sparse production. This is called alternate bearing. You
can reduce the potential of a tree to alternate bear by reducing the fruit load on a heavy fruit set year by
thinning out some of the fruit. Pruning the tree will also help to offset alternate bearing. Also, fertilize less
in light years and more in heavy years so that the trees needs are met according to the demands of the fruit
load. Lastly, do not allow the old fruit to stay on the tree longer than necessary. Despite using these
strategies, some varieties will just alternate bear.
11. How can I control weeds under citrus?
YOUNG TREES: To encourage good growth, a weed or grass-free area of at least 4 feet diameter should
be maintained around young trees. Landscape fabric covered with bark or rock around the young tree
will help. Careful and proper use of the herbicide glyphosate (Roundup®) can control weeds but must be
kept off the trunk and leaves. Do not use glyphosate near Star Ruby nor Flame grapefruit trees. MATURE
TREES: a grass-free area out to the drip-line is desirable but not essential (see Q. 25). Read all
pesticide/herbicide labels carefully, and use according to the manufacturer's directions for best results.

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12. Should I sprinkle citrus foliage?
Spraying the foliage occasionally to wash off the dust is okay, but it is best to keep the tree trunks dry
(see Q. 32, 33). Also, citrus foliage is sensitive to salt. Colorado River water contains fairly high
concentrations of salts and may harm the leaves if they are wetted regularly.
13. Should pruning wounds be treated?
Research has shown that you should never use a tree seal. Small pruning wounds (1 inch or less) need not
be treated. Tree seals tend to seal in disease and interfere with the tree’s natural ability to callous the
wound. For large pruning cuts lower in the canopy you may treat with Bordeaux mixture (See recipe in
Q. 33).
14. Should I spray the trees with 2,4-D? When?
This chemical is used at times in commercial citrus groves, but it is not recommended for backyard tree
use. Incorrect use could result in severe injury to the trees or other nearby plants.
15. How close together can I plant my citrus trees?
Mature trees on well drained soil will have a canopy diameter of 18 to 30 feet depending on the variety.
Lemons and grapefruit are the largest and mandarins are the smallest.
Commercial groves generally plant trees on 12 x 24 ft. spacing. That is, 12 ft. between trees going down
the row and 24 ft. between rows. When the tree size causes too much shading for the lower limbs
(excessive shade reduces production), trees are pruned and/or hedged.
“Dwarf trees” are regular scion varieties grafted onto ‘Flying Dragon’ rootstock. In general, trees grown
on ‘Flying Dragon’ rootstock are much smaller than the same varieties on other rootstocks. Spacing for
these trees can be as close as 6 x 12 ft. Also, trees planted on heavy soils will not grow as large as trees
planted on well-drained soils.
16. When can I bud graft my trees?
Bud grafting is most successful when the trees are naturally pushing new growth. Preferably in the spring
when temperatures are warm (February-May), growth “is on” and the bark “slips” easily. Scion to
rootstock grafting may be done any time but it is best to avoid times when temperatures are low and there
is a risk of frost.
17. Should I water at blooming time?
Drought during bloom causes flowers to drop and results in poor fruit set. To optimize fruit set and
minimize fruit drop, always maintain good irrigation practices throughout the year.
18. How do I protect my citrus from frost?
Frost risk is defined as conditions when winter temperatures fall to 29°F for 30 minutes or longer. These
conditions can cause some damage to tender plants. Citrus varieties vary in their sensitivity to cold.
Satsuma mandarins and kumquats are among the cold-hardiest. Oranges, grapefruit and mandarin hybrids
are intermediate in their tolerance to frost. Limes and lemons (except for Meyer lemon) are most
susceptible to frost damage. Healthy trees that are well supplied with water are better able to withstand
frost than weak, dry trees. Avoid placing citrus in the lowest areas of the garden, as cold air flows
downhill to the lowest point (in the same way that water flows downhill). To protect young frost-sensitive
trees wrap them with insulating material, such as palm fronds, corn stalks, or cardboard. Cover the trunks
from the ground level up to the main branches. When frost is expected, keep the soil surface below the
tree clean and wet as this will act as a heat sink. When severe frost is anticipated small outdoor holiday
lights can be strung in the tree. Frost will rarely kill a mature citrus tree. If the leaves or twigs show signs
of frost damage, be patient and wait until the spring flush of growth to determine what to prune off.

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19. When should I fertilize citrus trees?
Apply one application of nitrogen (ammonium nitrate, ammonium sulfate, or urea) in late December to
February. Alternatively, fertilizers can be applied in several applications. Many commercial growers
apply 1/3 of the total nitrogen needed in February, July, and September. Using a balanced citrus food may
help to correct mineral deficiencies as well as provide a more complete nutrition. Manure should be
applied in the fall (October-November) so that the winter rains can leach the salts (see Q. 23). Steer and
chicken manure should be used sparingly because they are high in salt and may burn the trees. Mature
citrus trees use 1-1.5 lb. of actual nitrogen (N) per tree per year (see Q. 21).
20. How much phosphorus do citrus trees need?
Phosphorus requirements for fruit trees are small. About 1 ¾ cup of phosphate fertilizer should be mixed
with the planting soil when the tree is planted. Then every 3 to 4 years, add about 1 pound of phosphate to
the soil around the root system of the tree. Work the phosphate into the top 1 inch of soil. Be careful to
avoid excessive root damage. Citrus feeder roots are primarily within the top 6-8 inches of soil. Root
injuries weaken the tree and may introduce root diseases. For best results, phosphorus should be applied
to the soil and not the foliage.
21. How should trees be fertilized; how much fertilizer should be applied?
A. YOUNG TREES (2-3 YEARS): Two tablespoons of nitrogen spread under the tree prior to irrigation.
Repeat 3-4 times each year. Double this amount the third year. About one gallon of good, composted
manure can also be used in place of chemical fertilizer. Mix the manure with the soil under the tree.
Remember that manure can be high in salts (see Q. 23).
B. MATURE TREES: One to 3 pounds of actual nitrogen/tree/year. Scatter over root area of tree (under
tree and 1-2 feet outside the drip-line). Then water it into the soil. Annual applications of ½ inch of
well composted manure in the trees drip-line may be used in place of commercial fertilizers.
General nitrogen (N) requirements for main citrus types:
Mandarins and oranges: 1-1.5 lb N/year
Grapefruit: 1-1.5 lb N/year
Lemons: 2-3 lb/year
To calculate the amount of nitrogen the first number in the parenthesis following the fertilizer name is
the percent nitrogen in the fertilizer. For example, ammonium sulfate (21-0-0) has 21% N.
Therefore, for every 5 lb of ammonium sulfate applied, the tree receives 1.05 lb of actual nitrogen.
Here is a list of fertilizer formulations and the amount needed:
5-10 lb. per tree of ammonium sulfate (21-0-0) = 1 to 2 lb. actual N
3-6 lb. of ammonium nitrate (33-0-0) = 1 to 2 lb. actual N
2-4 lb. urea (46-0-0) = 1 to 2 lb. actual N
22. There are a considerable number of small fruits dropping. Why?
This is frequently referred to as “June drop”. Young fruit (smaller than 1 inch in diameter) may drop in
May, June and/or July. Some fruit drop is natural. Excessive drop may be due to drought stress, sudden
high temperatures, low humidity, or nitrogen deficiency. Heavy pruning, thrips, mites, or spray injury can
also cause fruit to drop. Keep trees in good health and well irrigated to minimize fruit drop. Fruit drop is a
self-regulating mechanism in citrus trees. Too much fruit set will cause small fruit size. Additionally,
excessive fruit set can also be damaging to trees (see Q. 8).

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23. I applied manure to my citrus tree. A few days later the foliage turned yellow and a
number of shoot tips died. What happened?
Manure can be high in salts. Salt can be damaging to the roots and can result in yellow leaves, leaves with
the leaf margins that are necrotic (dead), or in shoot tip death. When manure is applied over the root
system the salts are carried into the root zone with the water and cause injury to the tree. Irrigate the soil
thoroughly to leach the salts out of the root zone. Use manure sparingly (see Q. 19). Manure that is not
well composted may release ammonia that may cause direct injury to the roots. Always use well-
composted manure and apply it the fall so that the salts can be leached by winter rains while the roots are
less actively growing.
24. What is iron chlorosis of citrus and how can it be corrected?
Iron deficiency is rarely found in Riverside, San Bernardino or San Diego Counties, see Q. 26. Iron
chlorosis is characterized by a yellowing of leaf tissues between the veins. The veins usually stay green
except when the deficiency becomes severe, then the new leaves turn completely yellow and dieback may
occur. Alkali and excess salinity contribute to iron deficiency. Iron deficient trees grow poorly and
production suffers. Proper irrigation may promote more vigorous growth and therefore may be the best
treatment (Q. 29). Rootstocks vary considerably in their ability to pick up iron. In the desert, trifoliate
orange typically will have iron deficiency and therefore other rootstocks are better suited under these
conditions. Zinc and manganese deficiencies are much more common and one deficiency may actually
mask another. Therefore, the best course of action for backyard citrus is to treat for all micronutrients.
(See also Q. 26.)
A. Use a micronutrient spray that contains iron, zinc, and manganese. Follow the directions for use on
the label. Foliar nutrients are not taken up through the older leaves and therefore sprays must be
applied to a new flush of leaves that are at two-thirds of their full size in spring and late summer.
B. There are several foliar sprays that contain iron that may be used to cure iron chlorosis. The addition
of a wetting agent to the spray is helpful in promoting good coverage. Use caution with foliar sprays.
Chelated iron is very acidic and can cause leaf burn. Several sprays of a dilute solution may be
preferable to one at full label rate.
C. Chelated iron or sequestrene iron may be spread on the soil under the tree and cultivated or watered
in. Desert soils tend to bind up iron, so results from this treatment are slow and may be poor.
25. My trees grow in a lawn. Do they get enough nitrogen when I fertilize the grass?
Probably not. The area under the tree should be fertilized more often than the lawn to ensure adequate
nutrition for the tree. The grass uses most of the nitrogen applied. When leaves of the trees show a slight
yellowing (paler green) they can be sprayed with one ounce (2 heaped tablespoons) of urea (46-0-0) or
low biuret urea (formulated for foliar application) in one gallon of water. Do not exceed this amount or
you may burn the foliage. Also do NOT spray urea during HOT weather. It is better not to grow citrus
trees within a lawn area - it is difficult to irrigate and fertilize both adequately, and generally the tree will
suffer (see Q. 11).
26. The leaves on my tree have green veins and are yellow between the green veins. Is this
iron chlorosis?
The most common nutrient deficiencies in Southern California are zinc and manganese. Iron chlorosis is
rarely a problem, especially in Western Riverside County (see Q. 24). Zinc deficiency causes “mottleleaf”
or “little leaf” with small terminal leaves and yellow mottling between the large leaf veins. Dieback of
twigs may occur in severe zinc deficiencies. Manganese deficiency is similar to zinc, however, the area
between the veins is a pale green with darker veins. These deficiencies are easily corrected with a
zinc/manganese spray to the foliage whenever there is a new leaf flush. For best absorption, apply when
the new flush of leaves is at two-thirds of full expansion. There are several commercially available
zinc/manganese combination fertilizer products that can be used to treat both zinc and manganese at the
same time. Be sure to treat for zinc deficiency at least 6 weeks before or after any phosphate fertilizer is

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applied. Zinc and phosphate cannot be absorbed by the plant at the same time and therefore, the zinc
treatment will not be effective if the plant has been recently fertilized with phosphorus. Carefully read and
follow label directions.
27. Why does the pulp of some citrus fruit dry out?
A. Developing and mature fruit are a source of water for the tree. Therefore, if the plant is water stressed,
it may withdraw water from the fruit causing it to dry up (granulation). Keep trees adequately watered
while the fruits are developing. Fairchild mandarin is very susceptible to granulation, particularly
with the larger fruit sizes.
B. Frost injured fruit will dry out on the tree.
C. Fruit that is mature and allowed to hang on the tree into the summer will tend to dry out and get
28. The limes on my tree have turned yellow. Are they really lemons?
Limes turn yellow when they are fully ripe (mature). There is nothing wrong with the fruit. Consumers
expect limes to be green and thus they are picked green. All limes will turn yellow when ripe. They are
still very good. Limes are best picked for that unique lime flavor when just a blush of yellow shows on
the skin of the fruit.

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29. How much water does my tree need per day?
Table 1. Riverside Area
Average Irrigation Water Requirements**
**Assumed application efficiency of 75% with values based on long term average water use. Figures in Table 1
may be higher during hot and windy weather.
Table 2. Desert Area
Average Irrigation Water Requirements**
**This table represents the average daily water requirements for citrus in the Riverside and San Diego County
desert areas. Irrigation system emission uniformity is NOT included in these figures. These figures are intended to
be used as a GUIDE and are based on average weather conditions.

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30. The leaves have been gradually dropping from my tree, the leaves on the tree are yellow,
the canopy is so thin I can see the sky through it and the leaves tend to be very small.
What is going on?
Phytophthora spp. can cause a root rot. Phytophthora root rot causes a slow decline of the tree. The leaves
turn a light green or yellow color and may drop, depending on the amount of infection. The fungus decays
the roots making them unable to take up sufficient water and therefore causing water stress and nutrient
deficiency symptoms. As the tree is wilting more water is often added causing the fungus to infect more
roots. Roots infected by Phytophthora spp. are brown (healthy roots are light tan). The outer portion of
the root sloughs off easily leaving the central portion (stele) of the root intact. When digging up the roots
(top 6 inches of soil) of a tree infected with Phytophthora spp. it will be difficult to find roots. One
indication of root health is to lightly scrape your fingernail along the root. On infected roots, the root
cortex (outer covering) is easily separated from the stele (central core). Provide adequate soil drainage
and avoid over-irrigating (see Q. 29). If destruction of feeder roots is minimal, corrective action may
include irrigating more frequently with less water so that the soil does not become oversaturated. For
other questions related to Phytophthora or gummosis caused by Phytophthora spp., see Q. 33.
31. Should soil accumulate around the trunk?
No. The base of the trunk should be exposed to the air and kept dry. If the trunk is allowed to stay wet,
this predisposes the tree to a fungal disease, Phytophthora root rot and Phytophthora gummosis (see Q.
30, 33). These diseases may kill the tree. Rodent injury is also common to tree trunks where soil and
refuse have accumulated. This may also predispose the tree to dry root rot. (See Q. 36).
32. Should irrigation water come in contact with the base of the trunk?
No. Water in direct contact with the base of the trunk may encourage root and trunk diseases (see Q. 30,
31, 33). One of the better methods of irrigating citrus is to use two microsprinklers with half-circle to
three-quarter emitters. Make certain that the emitters are adjusted so that the trunk is kept dry. If
microsprinklers are not an option, then dig an irrigation circle (doughnut or moat) 2-3 inches deep around
the base of the tree, a few feet from the trunk, and water outside this ring but be sure not to expose or
damage roots when making the basin.
33. The base of the trunk or lower limbs secretes clear gum. The bark dries upward, hardens
and cracks. What is this and what can I do?
This may be gummosis, a fungal disease caused by Phytophthora spp. It can be controlled, if diagnosed
early (before 50% of the circumference of the tree has a canker). Remove soil from base of trunk so the
tissue can dry. Do not allow irrigation water to come in contact with the trunk (see Q. 32) and do not
over-irrigate (see Q. 29). Be certain that the graft union (the point where the scion or cultivar is grafted
onto the rootstock) is not in contact with the soil. The graft union can be identified by looking for a line
around the trunk of the tree with a slight dogleg shape or bulge just below the line. The graft union should
be several inches above the soil level. Most scions are very susceptible to gummosis and will quickly
succumb to this disease if in direct contact with the soil. Apply Subdue®, Ridomil® or Aliette® to the
lesion or canker. Another approach is to remove dead bark and/or paint the lesion area with Bordeaux
paint. Garden stores may have a prepared Bordeaux mixture. If not, you may prepare Bordeaux mixture
paint by mixing equal parts of copper sulfate and hydrated lime with enough water to make a slurry. Read
and follow label directions carefully. Keep the trees growing as vigorously as possible with regular
watering (do not over water). Also, proper fertilization will promote growth.

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34. Some of the limbs on my tree are oozing copious amounts of sap and are declining. Small
areas of bark dry up and flake off leaving wounds on the trunk and lower limbs. What
causes this and can it be controlled?
One possible cause is Rio Grande Gummosis. The cause of this disease is unknown but is thought to be
caused by a fungus (see Q.35). This disease is most common in Imperial and Coachella Valleys on
grapefruit and lemon trees. It is often associated with pruning of large branches. To help control the
disease, remove the dead bark, clean the wounds and paint the trunk and lower limbs with Bordeaux
mixture (see recipe Q. 33). Use a paintbrush to apply the slurry to the cleaned wounds.
35. Some branches on my citrus tree dry up. When the bark is examined a black, sooty
powder is seen. What causes this disorder?
The branches may have branch wilt. This is not common and is a disease caused by a fungus which
usually attacks trees that are in poor health or are under some form of stress (drought, salt injury, poor
nutrition). There is no chemical control. Infected limbs should be cut off and destroyed (burned). Trees
should be fertilized and watered to maintain healthy, vigorous growth. If soil is high in salts, select tree
species that are tolerant to salty conditions.
36. The bark on the trunk at or near the soil line dries out and cracks. The wood under the
affected bark is stained grayish brown to purple. What is the problem?
The disease is “dry root rot”, caused by the fungus Fusarium solani. Unlike Phytophthora gummosis, dry
root rot does not produce gumming on the trunk and the lesion extends deep into the wood. The initial
infection may occur at the time of planting or at any time during the life of the tree, however,
aboveground symptoms may appear several years after the initial infection when the crown of the tree has
been girdled. Once the crown is girdled, the tree collapses. The wood below the dead bark is dry, hard and
stained grayish brown to purple. Infection occurs through injured tissue caused by gophers, Phytophthora
spp., mechanical injury, or root burn (caused by a large overdose of fertilizers or herbicides). All common
rootstocks are susceptible to dry root rot. To prevent dry root rot, keep base of trunk free of soil and dry.
Control gophers. Keep trees adequately fertilized and in good vigor to suppress this disease.
37. Why are my fruit dropping just before harvest? The fruit appear normal from the outside
but there is black rot extending from navel to the center of the fruit.
This is a fungus disease called Alternaria black rot. The fungus causes premature drop of nearly mature
fruit, mainly Navels, but can occur in grapefruit and other citrus. The fungus enters through the navel in
Navel oranges, or though the button in other varieties. It is wind-borne and tends to be a problem when
the trees are stressed (i.e., during Santa Ana wind conditions). It occurs more frequently when warm
rains follow severe winds. There is no control.
38. Why are my fruit splitting just before they are ripe?
Fruit splitting is most common on thin-peeled fruit such as Navel oranges. It generally occurs following
stresses such as extreme hot weather combined with high winds or drought stress followed by heavy
irrigation. In both cases, the tree becomes drought stressed and begins to withdraw water from the fruit.
When the tree is irrigated the dehydrated fruit swell causing them to split. The best remedy is to irrigate
evenly (Q. 29). When severe winds or high temperatures are anticipated irrigate before the weather
change occurs. Following the hot winds irrigate lightly for a few days and then resume a normal irrigation

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39. My orchard is on recently cleared land. The trees began growing well but have developed
bark damage at the base with a white film under the bark and a distinct “rotting” odor. There
are ants present at the base of the tree. Mushrooms grow from the ground in the grove after
rain. What is happening?
This disease is called Oak root rot, or Armillaria root rot and is caused by the fungus, Armillaria mellea
or the honey fungus. The fungus occurs naturally where oaks were native. When clearing the land,
remove as much of the roots and debris from native vegetation as possible. Leave the land fallow for a
year or more and then plant the trees. There is no control once the tree is infected. If the tree is infected
with Armillaria do not replant with citrus.
40. What causes leaves to become yellow?
There are several causes. The most common cause is nitrogen deficiency (see Q. 21). Additionally,
excessive watering leaches nitrogen fertilizer so that the tree is unable to take up the fertilizer. High lime
content in the soil can cause iron, zinc, and manganese deficiency (see Q. 24, 26). Girdling of the trunk
by rodents as well as Phytophthora root rot (Q. 31, 33). Sunburned leaves appear yellow. Sunburning of
the leaves occurs when the tree is under irrigated, causing the leaves to cup, exposing the more sun
sensitive lower leaf surface. Some grapefruit trees develop a yellow color in the early spring months
(called spring or winter chlorosis), common in young ‘Rio Red’ grapefruit trees. Grapefruit trees with
winter chlorosis will outgrow the problem. Star Ruby grapefruit have a yellow streaks in the leaves. This
is a characteristic of the variety.
41. Why is the bark of the lower trunk missing?
Field mice or gophers may be the culprits. However, where trees are planted in a lawn, string trimmers
(weed whackers) can beat the bark off of young trees and may kill them. Lawn mowers may also cause
similar damage. Keep the area around young trees free from grass for at least one foot (preferably 4 feet)
from the trunk to avoid these problems (see Q. 11, 33).
42. Why are wide areas of the trunk or limbs dried up and cracked? Furthermore, why has the
bark became detached?
Frost and sunburn are common causes for this type of damage. Sunburn usually occurs on the south and
southwest side of limbs and the trunk. Use whitewash to prevent damage (see Q. 5). Frozen plant parts
will also dry, crack and have peeling bark. Navels, Temple tangors and lemons are very frost sensitive.
See Q. 18, 48.
43. Which are the most common physiologic, disease, and insect problems?
• Leaf yellowing due to excessive soil salts or excessive irrigation (Q. 24, 26, 40).
• Dieback and leaf death due to frost or salt injury (Q. 12, 23, 30, 35, 40).
• Death of tree due to frost (i.e., Temple tangors and Navel oranges in some cases, Q. 8) and
Phytophthora gummosis (Q. 30, 33).
• Iron and zinc chlorosis (Q. 24, 26, 40).
• Fruit drop and splitting due to irregular irrigation and hot weather (Q. 22, 38).
• Phytophthora root and trunk rot (Q. 30, 33).
• Rio Grande gummosis of grapefruit. Pruning cuts or damaged limbs ooze excess frothy gum (Q. 34).
• Leaf and fruit scars due to citrus thrips, mites (Q. 45, 46).
• Dry-rot trees suddenly die and when cut the wood is stained grayish brown to purple (Q. 36).
• Red scale (ask for circular 127, see Q. 44).
• Termites which enter through dead limbs and then damage live wood.
44. What should be applied to control red scale and when should it be applied?
Spray with Malathion
or Sevin
follow directions on the label. Apply sprays after bloom is completed or
in July. Do not apply any sprays during bloom. Apply light grade 415 or 440 oil in August to September.

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Biological control may be possible with the release of insects that are parasitize red scale. These parasites
can be purchased from insectaries. The best parasite for red scale is Aphytis melinus.
45. How do I control mites and when?
Apply sulfur before trees bloom, usually mid to late February. 415 or 440 oil petroleum spray applied in
August will also help suppress mites. Do not use vegetable oil.
46. The leaves on my tree are deformed, thick, and small. There are rings of scars on the
fruit. What is causing this damage?
Thrips can severely damage citrus causing deformed leaves and scars on the fruit. Mandarins and oranges
are most severely affected. Additionally, younger trees are more susceptible to damage and are more
attractive to thrips. Organically acceptable methods of control are: Sabadilla (Veratran D), Ryania, or
biological control with Euseius tularensis or Anystis agilis. Thrips do not hurt mature trees but can cause
scarring on fruit. Scarred fruit are blemished yet edible.
47. How do I control snails?
Snails can cause severe damage to the trunk, fruit and foliage of a tree. In the desert, snails may be
unsightly but generally do not harm the tree. A good snail management program relies on a combination
of methods.
A. Use copper barriers such as copper flashing, screens, foil or paint. Copper reacts with the slime that
the snail secretes, causing a flow of electricity that becomes the barrier to the snail.
B. Biological control with decollate snails is also very effective. Decollate snails are effective predators
of juvenile snails, so that brown snail adults cannot be replaced as they die off. Once established,
decollates will nearly eliminate the brown garden snail population in four to six years. Decollate
snails are only legal to use in the eight Southern California counties. For effective control introduce
decollates in as high a number as financially feasible. The best time to release decollates is when the
temperatures are above 50 F°. Decollates will establish best in moist environments and can flourish if
they have sufficient food in the form of leaf litter and fallen fruit.
48. Why is the bark on the trunk of my tree scaly and flaking off?
Psorosis is a graft transmissible disease, caused by a viroid (virus-like), most often found in old citrus
plantings. It is transmitted in infected budwood or possibly with contaminated grafting tools. This disease
generally occurs on trees planted before the University of California Riverside Citrus Clonal Protection
Program was initiated to provide disease-free budwood (early 1960’s). Infected trees, mostly orange and
grapefruit, slowly decline. The trees gradually become unproductive. The most distinguishing field
symptom is scaling and flaking of the bark on the trunk. Clear gumming may appear on the branches.
Eventually main scaffold branches will die. Where an old tree shows symptoms, scrape away the infected
bark area to stimulate the formation of wound callus, this results in temporary recovery. Eventually the
tree becomes non-productive and should be removed. There is no cure for an infected tree.
49. All of a sudden my citrus tree dropped all its leaves and died. What caused this?
Most trees are not planted on susceptible rootstock, however, if your trees are grafted on a rootstock,
such as sweet orange, it may have quick decline. Quick decline is caused by a virus, Citrus Tristeza virus,
which can be fatal to citrus. The leaves curl up along the length of the midrib and take on a dull ashen
color before suddenly falling to the ground. There is no known cure for Citrus Tristeza virus. This
disease is not common because nurseries typically use resistant rootstocks. Other potential causes for
sudden tree death are dry root rot (See Q. 36) and Phytophthora root rot (See Q. 30, 33). Also, some
mandarins and mandarin hybrid crosses can die suddenly after a heavy fruit set (See Q. 8).
50. Any further questions?
Call the Master Gardener Program Hotline at (909)683-6491 – Monday through Friday, 9 to 12 noon.

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Integrated Pest Management for Citrus, Second Edition. 1991. University of California Division of Agriculture
and Natural Resources, Publication 3303. 144 pgs.
Walheim, Lance. 1996. Citrus – Complete guide to selecting and growing more than 100 varieties for California,
Arizona, Texas, The Gulf Coast, Florida. Ironwood Press, Tucson, AZ. 112 pgs.
Useful UC websites:
UC Integrated Pest Management:
Alternative subtropical crops:
UC Citrus Clonal Protection Program:
UCR Citrus Variety Collection:
UC Pest Notes:
UCR Entomology – Biocontrol:
UC Fruit & Nut Center:
UC Postharvest Information:
UC Small Farm Center:
UC Sustainable Ag Res. & Ed.:
UCR Botany & Plant Sciences:
UCR Plant Pathology:
UC ANR catalog:
UC Ag Economics Cost studies:
UC Cooperative Extension – Riverside County:
12/18/02 TS/pm

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INDEX (referenced by page number)
Alternate bearing · 4
Bud graft · 5
Chlorosis · 7, 12
Diseases · 10-13
dry root rot · 10, 11, 13
oozing · 11
tree dying · 4
Dwarf citrus · 5
Fertilization · 6-8
amount · 6, 7
chlorosis · 7, 12
manure · 6, 7
timing · 6, 7
types · 6, 7
Frost · 3, 5, 8, 12
black rot · 11
drop · 5, 6, 11, 12
dry · 8
holding on tree · 4
June drop · 6
limes, yellow · 8
split · 11
Grafting · 5
Granulation · 8
Grapefruit · 1
Gummosis · 4, 10, 11, 12
mites · 13
scale · 12
thrips · 13
Iron · 7, 12
daily requirements · 9
foliar · 5
placement · 10
timing · 5
Kumquats · 3
chlorosis · 7, 12
yellow · 10, 12
Lemons · 1
Limes · 1
Mandarins · 2, 3, 4
Manganese · 7
Minor elements · 7
Mites · 12
Mushrooms · 11
Nitrogen · 6
Oozing · 11
See also Gummosis
Oranges · 2
Blood orange · 2
Navel orange · 2
Valencia orange · 2
Phosphorus · 6, 8
depth · 3
spacing · 5
timing · 3
tree size · 3
Pollination · 2, 4
Pruning · 4, 5
timing · 4
Psorosis · 13
Quick decline · 13
Red scale · 12
References · 14
Rodents · 10, 12
Root rot
Armillaria · 12
dry root rot · 11, 12
10, 11, 12, 13
Satsuma · 2
Scaly bark · 13
Site selection
lawns · 7
soil type · 3
Snails · 13
on trunk · 10
type · 3
Splitting, fruit · 11, 12
Sunburn · 4, 12
Tangelos · 3, 4
Tangerine · 2, 3, 4
Tangors · 4
Thrips · 12, 13
Tristeza virus · 13
bark missing · 12
gumming · 4, 10, 11, 12
oozing · 11
protection · 4
Grapefruit · 1
Lemons · 1
Limes · 1
Mandarin · 2
Oranges · 2
Satsuma · 2
Tangerine · 2, 3
Weed control · 4
2,4-D · 5
White wash
recipe · 4
Zinc · 7, 8, 12

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