GUMMOSIS OF CITRUS TREES

 

 

 

ITRODUCTION

 

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.  This 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 trunk 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 susceptible. Foot rot 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 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 this disease .

 

 

SYMPTOMS

 

An early symptom of phytophthora gummosis is sapoozing from small cracks in the infected bark , giving the tree a bleeding appearance .The gumming may be washed off during heavy rain. The bark stays firm , dries , and eventually cracks and sloughs off .Lesions spread around the circumference of the trunk , slowly girdling the tree .

Decline may occure rapidly within a year , especially under conditions favorable for disease development , or may occur over sevral years .

 

 

COMMENTS ON THE DISEASE

 

Phytophthora fungi are present in almost all citrus orchards .Under moist conditions , the fungi produce large nombers of motile zoospores , which are splashed onto the tree trunks .The Phytophthora species causing gummosis develop rapidly under moist , cool conditions . Hot summer weather sloes disease spread and helps drying and healing of the lesions .Secondary infections often occur through lesions created by Phytophthora . These infections kill and discolor the wood deeper than gummosis itself.

 

 

MANAGEMENT

 

The only really effective method of control for Phytophthora is prevention. Once the fungus is introduced and becomes established in a grove or nursery, it is very difficult or impossible to eradicate. Preventative measures, therefore, have to be observed during all the stages of citrus production.

Nursery stage
All nursery practices have to consider sanitation as the main step for preventing Phytophthora infections, as well as other diseases. Firstly, the seeds used should be from certified sources and should be properly surface disinfected. The planting medium should be disinfected as well, including the germination mix and the bagging mix.

                 At least the bagging mix should come from an area where no citrus has previously been planted. The nursery site should also be well drained and should allow minimum access from outsiders. It is strongly recommended to have a disinfection pit at the entrance to disinfect shoes. 

                Budding practices are as important as management of the plants. It is recommended that budding be done no lower than 12 inches above ground level on the bag. 

 

Land selection
         
One of the main factors to consider in planting a new site is the drainage characteristics of the soils. Poorly drained soils either because of low lying topography or because of poor water conductivity due to high clay contents are not suitable for citrus.

            These will be conducive to future problems with Phytophthora. Farmers should select well drained areas or improve drainage capabilities by putting drainage ditches in place. Planting citrus on cambered beds is a very effective way of preventing heavy losses from the disease. Cambering keeps the root system above the water table during periods of high rain fall.

Varietal resistance
Rootstock varieties of citrus differ greatly in their degree of tolerance to Phytophthora. Some varieties are highly tolerant while others are highly susceptible. Susceptibility may vary with age and be highly determined by soil and environmental conditions. There is also a major difference between foot rot susceptibility and root rot susceptibility. Some varieties may express good foot rot tolerance but be root rot susceptible.

 

 

 

REFERENCES

 

 

1) Fungal Diseases and Nematodes
Jose M. Amador
Extension plant pathologist
The
Texas A&M University System

 

أمراض النبات- د.توفيق مصطفى العنتري 2)

 

3) Brown Rot in Arizona Citrus
Mark Wilcox,
Yuma County Cooperative Extension.
Volume 2, Issue 1. September, 1994

 

4) SITES

http://www.apsnet.org/pd/summaries/djn02sum.asp

www.apsnet.org/pd/pdfs/2002/0401-01R.pdf

www.doacs.state.fl.us/pi/budwood/definitions.html

www.ci.gilbert.az.us/water/popups/citrus.cfm