Amador, Jose M.

English

Texas A&M University

L-2313
Texas Agricultural Extension Service
Texas Citrus
Fungal Diseases and Nematodes
Jose M. Amador*
Citrus diseases affecting the entire tree can be
classified into two general categories: parasitic
and nonparasitic. Parasitic diseases are caused by
fungi, bacteria, viruses and nematodes. Some
disease-causing agents are restricted to certain
parts of the plant, such as the root, trunk or fruit,
while others may affect several or all parts of the
plant.
Nonparasitic diseases are physiological disorders
caused by excesses or deficiencies of certain
nutrients or by unfavorable environmental condi-
tions. Among the most common nonparasitic
diseases of citrus occurring in Texas are iron
chlorosis, mottle leaf, salt and chronic water table
injury. In addition to the economic losses that can
result if not avoided or corrected, these physio-
logical disorders often predispose the tree to
attack by disease-causing organisms, such as the
relationship that exists between salt and chronic
water table injury and the increased occurrence of
several types of gummosis, twig blight, tip-burned
yellow leaves and nematode infestations. Physio-
logical disorders are discussed in other sections of
this publication.
Fungus Diseases
Cotton Root Rot
This disease, caused by the fungus Phymato-
trichum omnivorum, affects many plant species.
The fungus attacks the underground parts of the
citrus tree and occasionally kills young trees. Once
trees reach maturity, they are less susceptible to
attack. Cotton root rot commonly causes sudden
death of susceptible trees, with most of the dried
*Extension plant pathologist, The Texas A&M University System.
leaves temporarily retained on the tree. The best
protection against cotton root rot is to use resistant
rootstocks such as sour orange.
Young citrus tree killed by cotton root rot.
Rio Grande Gummosis
This is one of several well-known gumming
diseases of citrus. Gum formation on the trunk or
branches is a characteristic symptom. Gum exudes
from blisters containing gum pockets, usually
located on the trunk. The wood beneath the blister
shows a pink-orange color.
Several factors such as freeze damage, high
water table and salt accumulation contribute to the
disease. Gummosis is believed to be a condition of
weak and injured trees and is reported to be
infectious. Symptoms of the disease were repro-
duced when healthy trees were inoculated with the
fungus Diplodia natalensis.
No reliable cure exists for gummosis. Keeping
trees vigorously growing is the best way to avoid
the problem.
Texas Agricultural Extension Service. Zerle L. Carpenter, Director. The Texas A&M University System. College Station, Texas
~-------------------
Rio Grande gummosis causes amber exudate to
be secreted from pockets beneath the bark.
PhytQphthora Foot Rot
Foot rot, also known as brown rot gummosis, is
caused by one or more species of the fungus
Phytophthora. This disease can affect the root
system, the tru,nk below and above ground,
branches, leaves, blossoms and fruit. It is especially
troublesome during prolonged rainy periods. Trees
with the bud union beneath or close to the soil and
trees in poorly-drained locations are highly sus-
ceptible. Foot rot is commonly found in Lower Rio
Grande Valley orchards, but becomes a more
serious problem under unusual conditions such as
those that occur following hurricanes.
Infection of the lower areas of the trunk by
Phytophthora spp. results in dark, water-soaked
areas in the active areas of infection. Often gum
Gum exudation from lesions on the lower trunk
during the early phase of Phytophthora spp.
exudes profusely from active lesions. The dead
bark frequently breaks away from the wood in
vertical strips. Callus tissue begins to form on the
margin of the surrounding healthy bark if the
fungus becomes inactive because of unfavorable
weather conditions. The disease may become active
again when conditions become favorable. If the
lesion encircles the trunk, girdling occurs and
results in death of the tree.
Healing is slower if infection occurs below
ground level. The fungus may attack young feeder
roots, causing them to decay. Infection of lateral
and fibrous roots can become widespread in wet
soils. This. infection results in poor health of the
tree, a thin canopy, failure to make new growth
and poor fruit production.
Phytophthora spp. also may attack nursery stock
and young orchard trees during rainy weather.
Examination of the crowns of infected trees shows
symptoms similar to those described for older
trees.
Phytophthora foot rot can best be controlled by
preventative practices, including use of resistant
rootstock and planting in well-drained land. Sour
orange is the most resistant rootstock for the
Lower Rio Grande Valley. Other resistant root-
stocks include Swingle citrumelo and Troyer and
Carrizo citrange.
Budding seedlings high, avoiding wounds, and
keeping soil off the lower trunk are among recom-
mended control practices. Soil fumigation of
seedbeds should be practiced in field nurseries.
:"'" ._ J:; r. ~. _-:
Bark scaling on the lower trunk during advanced
stage of foot rot.
---------------~
Nematodes
Plant parasitic nematodes are very small worms
that cannot be seen with the naked eye. Several
plant parasitic nematodes are known to attack the
root system of citrus plants, especially the citrus
nematode, Tylenchulus semipenetrans. The bur-
rowing nematode, Radopholus similis, causes a
disease known as spreading decline and is a
serious pest of citrus in other production areas.
This nematode has been found in the Valley on
ornamentals, but not in citrus orchards. Serious
losses to the Texas industry could result should it
ever become established in citrus groves.
The citrus nematode occurs commonly in Valley
citrus orchard soils. Sour orange is rated as highly
susceptible to nematode infection. Of the common
rootstocks with potential use in the Valley,
only Swingle citrumelo is rated as highly tolerant
to the citrus nematode.
Populations of the citrus nematode usually are
higher in older orchards (6 years and older).
Young trees also may be injured if planted in
orchard sites where nematodes previously pre-
vailed. Other plant parasitic nematodes, mainly
the lesion nematode (Pratylenchus spp.) often
have been found associated with citrus roots.
Because the presence of the citrus nematode
cannot be detected visually, positive diagnosis is
based on external sympto'ms and laboratory exam-
ination of root and soil samples. Above-ground
symptoms associated with nematode-infected trees
are wilting, lack of vigor, poor fruit producton and
poor response to watering and fertilization. The
Texas Agricultural Extension Service operates a
Plant Nematode Detection Laboratory at College
Station where samples can be sent for analysis and
recommendations. County Extension agricultural
agents can assist growers in collecting and sending
samples.
A microscopic view of a swollen female citrus
nematode.
Losses caused by the citrus nematode in heavily
infected orchards can be reduced effectively by
applying nematicides to the soil. Before treating
an orchard, however, consider the overall condition
of the orchard. Application of nematicides to
freeze-damaged orchards grown under poor cultural
conditions may not be profitable. Soil should be
sampled before establishing a new orchard to
determine if nematodes are present in damaging
numbers. If the analysis reveals large numbers of
parasitic nematodes, treating the soil before estab-
lishing trees may be profitable. Ideally, nematode-
free plants should be used when establishing new
orchards.
At present, products cleared for use on estab-
lished citrus trees are also known to have an effect
on mites and other insects. Careful attention must
be paid to instructions on the label, not only for
directions on how to apply the material, but also
for current rates and regulations that may affect
the legal use of these products.
~----------
Acknowledgment
Appreciation is expressed to Pete Timmer and Mike Davis, former plant pathologists,
Texas A&I University Citrus Center, Weslaco, Texas, for photographs in this publication.
Educational programs conducted by the Texas Agricultural Extension Service serve people of all ages regardless of socioeconomic
level, race, color, sex, religion, handicap or national origin.
Issued in furtherance of Cooperative Extension Work in Agriculture and Home Economics, Acts of Congress of May 8, 1914, as
amended, and June 30, 1914, in cooperation with the United States Department of Agriculture. Zerle L. Carpenter, Director, Texas
Agricultural Extension Service, The Texas A&M University System.
1.5 M-10-88, New HORT 2-2, PP


Texas Citrus Fungal Diseases and Nematodes.

https://oaktrust.library.tamu.edu/bitstream/1969.1/175083/4/2313/Leaf2313.pdf

Texas Citrus Fungal Diseases and Nematodes.

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