Root rots

Root rots may be caused by Phytophthora cin-namomi and various Pythium species, with Pythium arrhenomanes Drechs. as the most common (Klemmer and Nakano, 1964). Initial symptoms are a reduction or elimination of growth, with subsequent reddening of the leaves, the leaf margins turning yellow and eventually becoming necrotic. With P. cinnamomi, which causes heart and root rot, the root-rot phase results in reduced plant growth and yields and, in cooler environments, can result in a total loss of the ratoon crop. Root-rot symptom development is relatively slow in comparison with heart-rot symptoms. Disease from both pathogens is most severe when soils are cold and poorly drained. If environmental and soil conditions become dry following the infection period, affected plants may appear reddish, as if under severe drought stress. Plant anchorage in the soil is very poor following loss of roots.

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