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Anthracnose (fungus - Colletotrichum gloeosporioides): Black circular spots up to one-half inch appear on fruit. The center of the spots may be slightly sunken and spots may develop cracks. During moist periods, the spots produce pinkish, moist masses of fungal spores. As the fruit ripens, the infection spreads rapidly into the flesh causing a greenish-black, fairly firm decay. Fungicides can control the disease and should be applied at bud swell.
Cotton Root Rot (fungus - Phymatotrichopsis omnivora): Although avocado is susceptible to cotton rot root, it is not a common disease. It is characterized by a sudden wilt of the tree. The leaves turn brown and remain on the tree. Very young trees tend to become diseased, rather than old ones. The only control is to avoid planting trees in soil known to be infested with the fungus.
See section on Cotton Root Rot
Leaf Burn (physiological): Leaves brown at the tips and the edges. Affected leaves may drop prematurely. These symptoms can be caused by accumulation of salts in the soil, inadequate soil moisture, wind dessication, and frost. Light irrigation should be avoided since this fails to leach accumulated salts out of the root zone. The soil should have good drainage.
Phytophthora Root Rot (fungus - Phytophthora cinnamomi): Infected trees have small leaves which are lighter green in color than healthy leaves. The leaves wilt and drop, eventually leading to complete defoliation of the tree. Twigs and branches die back. There is a light fruit set with small fruit. The fungus infects roots up to one-quarter inch in size, although it is the feeder roots that are primarily affected. These roots become blackened and brittle before they die.
The disease occurs if the pathogen is present in soil and if there is excessive soil moisture. Poor drainage can be a contributing factor to disease development. The fungus can be introduced to new areas by movement of infected nursery stock or infested soil. Soil on implements can also serve as a means for introduction of the fungus to non-infested areas. Fungicides should be used in combination with sanitation in the nursery and maintaining adequate drainage in the field.
Scab
(fungus - Sphaceloma perseae): Circular, brown, scabby areas are found
on mature fruit [Photo]. Leaves have brown spots and
become crinkled. The disease can become a problem if there is cool, moist weather
when fruit and leaf tissue is young. Fungicides can control the disease and
should be applied when flower buds appear, near the end of the main bloom period,
and 3-4 weeks after that.
Seedling Blight (fungi - Phytophthora spp.): This can be an important disease in nursery production. Leaves show irregular reddish-brown areas that enlarge along the larger veins. The terminal bud may be killed. The disease is favored by periods of heavy rainfall and high humidity, when seedlings are young and succulent. The following cultural practices can prevent or minimize damage. Plants should be grown on benches. The use of soil should be avoided to prevent introduction of the pathogen. Irrigation water from ponds may also serve as a pathogen source. Providing good air circulation between plants will provide a less-favorable microclimate for disease development. Fungicides should be used only in combination with cultural control practices.
See section on Seedling Blight
Sunblotch (avocado sunblotch viroid): Twigs have a light yellow, sunken streak that follow the lenght of the twig. Fruits have white or yellow areas. Trees are stunted and have a sprawling growth habit. The pathogen is transmitted in budwood, graftwood, or seed from infected trees. Disease-free nursery stock should be used and trees with symptoms should be removed from the orchard.
Wood
Rot (fungus - Ganoderma lucidum): This fungus contributed to the
gradual decline and death of trees in a portion of an orchard in the Rio Grande
Valley. The decline occurred over a period of several months during the summer.
The leaves of affected trees first turned pale to yellow and trees soon lost
leaves [Picture]. Eventually,
trees were completely defoliated and died [Picture].
Fruiting bodies of the fungus were found on dead and dying trees [Picture].
The above-ground symptoms of the disease resemble Phytophthora root rot. The
disease occurred in a portion of the orchard with poor drainage that was subject
to waterlogging. The waterlogging led to the death of feeder roots (again, a
symptom that resembles Phytophthora root rot) and stressed the trees. This environmental
stress allowed the fungus to develop within trees. The fungus infected the trees
via wind-blown spores that entered the tree through wounds. There is no chemical
control for this pathogen. Encouraging vigorous tree growth by minimizing environmental
stresses (e.g. improving soil drainage) will prevent establishment of the fungus
in the tree.
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