Symptoms of foot rot or gummosis often start appearing near the soil line. Dark water-soaked areas are formed in the bark and a sour smell may emanate from them in wet conditions. Water-soluble gum exudes from longitudinal cracks in the bark, especially in dry weather. Bark below the soil line turns water soaked, slimy, reddish-brown, or black in late stages. Brown necrotic areas can extend to internal tissues of wood. Foliage turns yellow due to nutrition deficiency. At later stages, the dead bark dries, shrinks and cracks and patches may fall off, leaving an open canker. Trees may collapse and die if the fungus encircles the bark. Infected fruits develop a soft brown rot that eventually produces a characteristic pungent odor.
The symptoms are caused by different species of Phytophthora fungi. They produce a large number of water-borne spores that under favorable conditions (high moisture and elevated temperatures) can swim over short distances. These spores are the infective agents that may be transported in rain or irrigation to the roots of the trees. They germinate and enter the root tip resulting in rot of the entire rootlet, later extending to the rest of the root. Foot rot or gummosis occur when these spores are splashed onto a wound or bark crack around the base of the trunk. The susceptibility of the tree depends on the Phytophthora species present and is greatly influenced by the prevailing environmental conditions (type of soil, presence of water).
Exposing the seeds to hot water (about 49 degrees) for 4-10 minutes eradicates seedborne infection. The insertion of chlorine into micro-irrigation systems effectively reduces Phytophthora infections. Some fungal or bacterial species (Trichoderma spp. and Bacillus spp.) have been tested as control agents for Phytophthora with good results. Copper fungicides can also be used to control the disease at early stages.
Always consider an integrated approach with preventive measures together with biological treatments if available. The treatment of the orchard with fungicides containing metalaxyl and fosetyl-aluminium is an effective complement to preventive and biological control of the fungus. Foliar applications of fosetyl-aluminium and a soil drench of metalaxyl showed very good results. Both preharvest sprays, postharvest dip treatments and/or the use of impregnated wrappers is recommended.
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