Organism: Phytophthora spp
This fungus cause the most serious soilborne diseases of citrus. The most serious is GUMMOSIS.
Wide range of fruit trees. Phytophthora fungi are present in almost all citrus orchards.
These fungi are worldwide in distribution and cause citrus production losses in irrigated, arid areas as well as in areas with high rainfall.
Symptoms and changeable diseases:
The bark stays firm, dries and eventually cracks and sloughs off. Lesions spread around the circumference of the trunk, slowly girdling the tree. Severely affected trees have pale, green leaves with yellow veins, a typical symptom of girdling. Decline may occur rapidly within a year, especially under conditions favorable for disease development, or may occur over several years. Infection of emerging seedlings by Phytophthora spp. causes damping off. Leaves and shoots near the soil may be blighted by Phytophthora spp., but in nurseries, only the tender new growth is usually affected.Secondary infections often occur through lesions created by Phytophthora. These infections kill and discolor the wood deeper than gummosis itself.
Phytophthora spp. infects the root cortex and causes a decay of fibrous roots. The cortex turns soft, becomes discolored, and appears water soaked.
Infection of the scion occurs near the ground level, producing lesions that extend down to the bud union on resistant rootstocks or up the trunk into the major limbs of the tree. Lesions spread around the circumference of the trunk, killing the cambium and slowly girdling the tree.
populations in the soil are maintained by repeated infection of the
fibrous roots. Under favorable conditions of high moisture and temperature,
the fungus produces sporangia that release motile zoospores, which are
attracted to the zone of elongation of new roots by nutrients that are
naturally exuded from this root zone. Upon contact with the root, zoospores
encrust, germinate, and then infect the area of the zone of elongation.
Once the fungus has entered the root tip, the infection may advance
in the cortex, resulting in rot of the entire rootlet. The cycle can
repeat itself as long as conditions are favorable and susceptible tissue
Phytophthora spp. most likely survive unfavorable periods in root debris. The rotted cortex is sloughed off, and the fungus produces chlamydospores, which may persist in the soil for long periods. When favorable conditions return, chlamydospores germinate indirectly to produce sporangia and zoospores or directly to produce mycelium. All species are also able to persist as mycelium or thick walled sporangia and as chlamydospores in infected, living roots
All scion cultivars are susceptible to infection under the right environmental conditions.
Management of Phytophthora gummosis focuses on preventing conditions favorable for infection and disease development.
- Resistant rootstocks are recommended for replanting in orchards with a previous history this disease.
- Budding high on the rootstock seedlings in the nursery and planting the trees in the orchard with the bud union well above the soil line avoids contact of the susceptible scion bark with infested soil.
- Adequate soil drainage is essential because poor drainage and over watering combine to promote build-up of pathogen populations in the soil, which increases the risk of bark infection.
- The soil surface under the tree must be kept free of weeds.
- Injuries to the trunk bark must be avoided, since they provide entry points for pathogens
In addition to improving the growing conditions, the spread of the disease can be halt by removing the diseased bark and a buffer strip of healthy, light brown to greenish bark around the margins of the infection. Allow the exposed area to dry out. You can also scrape the diseased bark lightly to find the perimeter o f the lesion and then use a propane torch to burn the lesion and a margin of 2'5 cm around it. Recheck frequently for a few months and repeat if necessary.
or postplant chemical control may be warranted if cultural controls
Systemic fungicides (Fosetil-AL) can control Phytophthora gummosis and copper sprays can be used to protect against infection.
may be applied to young trees on a preventive basis to control gummosis.
On tolerant rootstocks, fungicide treatments are used only if bark lesions
Fungicides should be applied during periods of susceptible root flushes, and soil applications should be targeted to areas of highest fibrous root density under the canopy.
If 50% or more of a trunk or crown region on a mature tree is girdled, it is more economical to replace the tree than to try to control the infection.