Cotton root rot is a soil-borne fungus which, fortunately, is not easily spread. It is incapable of movement through air or water, and requires direct contact, usually a diseased root contacting a healthy one, in order to spread. The fungus itself forms thread-like structures which themselves grow through soil looking for hosts. Even though it is slow to spread, its effect on its hosts plants is devastating and extremely rapid. P. omnivorum can over-take a mature tree in less than two weeks, and frequently within 72 hours of the first signs of infection. Other plants may just seem to die completely out-of-the-blue, often just after a period of growth.
The first sign is usually leaf yellowing. The discolouration progresses into a bronze colour. Wilt occurs, but leaves do not drop. Stems also wilt, eventually permanently. Reddish lesions may also be evident on the crown. The most distinctive characteristic of cotton root rot is in the soil. Roots are covered with the fungal threads by which the disease spreads, and the roots and base of the plant may be black and slimy.
This fungus only attacks mature plants, and mostly only dicotyledonous (broad-leaved) plants. Encouraged by soils low in organic material, the environs of Cacti and succulent collections are optimal for P. omnivorum. Alkaline soil also assists in its growth and spread.
There is no available treatment for cotton root rot, and the only effective control is to remove and destroy infected plants, along with their soil and containers. Since it does not spread easily, simply removing it from the area of healthy plants is usually effective.
PHYTOPHTHORA SPP. (cinnamon fungus)
Phytophthora is derived from the Greek words meaning "plant destroyer." One of the approximately 40 species of this fungus was responsible for the potato famine in Ireland during the 1850's, and its presence has also been an issue for mid-western farmers of peppers and cucurbits. It is a water mould, and is primarily soil-borne, although it can also be seed-borne. Warm, damp conditions are the most favourable for it, therefore poor drainage is usually the causal factor. Saline soil also encourages its growth.
Cinnamon fungus usually infects mature plants at or below the soil line, and then progresses upward. Root and basal stem rot are common, and roots may also show discolouration. The collar rot phase of the fungus causes dark-brown, water-soaked cankers on lower stems. These lesions continue to progress up the plant, and may weep black fluid. Leaves will have circular spots which are dark green and wet to begin with, becoming tan-coloured as they dry out. Wilting sometimes occurs before the fungus progresses above the soil line, and sometimes sans the appearance of cankers at all.
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