|
Powdery mildew
The disease has been reported from many countries: Cyprus, Ethiopia, India, Siberia, Sudan, Syria, Tanzania and the former USSR. In India, the disease occurs in severe form almost every year on different varieties. A similar situation has been reported from Sudan. In Syria, the disease is sometimes a problem on breeding material in plastic houses and in offseason nurseries, but it is rarely seen on the main crop in the field during the crop season.
The disease is caused by Erysiphe polygoni D.C. Its conidial stage, Oidium sp., has been frequently reported from different places. Leveillula taurica (Lev.) Arnaud. and its conidial stage, Oidiopsis taurica, have been reported only from the former USSR and Jordan. The first pathogen is an ectoparasite affecting plants at any stage of crop growth, but it is usually more severe at flowering. The second pathogen is an endoparasite.
Moderately high temperatures and moderate relative humidity favor the disease. The conidial stage is responsible for the disease spread. The perithecial stage has been reported to occur in India and Sudan.
Symptoms: A fine, powdery, white growth develops first as spots, then rapidly spreads to cover the entire surface of leaves, stems and pods (Fig. 17). Later, the leaflets become dry and curled and shed prematurely, causing considerable reduction in yield.
Control: Resistant lines have been reported from India in varieties from Iran.
Foliar spray with benomyl, tridemorph (I L/ha) or 3% aqueous sulfur (500 L/ha) can prevent disease development. Certain insecticides (Quinalphos, Tnazophos, Phoxim) have also been found effective in suppressing mildew growth.
Downy mildew
The disease has been reported from Egypt, France, India and Syria. Variable yield losses have been reported for some areas. However, if the spring is dry, yield losses caused by downy mildew are considered negligible.
The disease is caused by Peronospora lentis Gäumannn, an oomycete. The fungus overwinters in plant debris as oospores. The disease, spread by sporangiospores, is common and serious only under cool, wet and humid conditions.
Symptoms: Although all the aerial parts are affected, the disease may be confined to a few branches, the tips of which are often dwarfed and the leaves are twisted and curled (Fig. 18).
Chlorotic to yellow spots of various size and shape develop on the upper surface of leaves (Fig. 19). Under humid conditions, fine, dirty pinkish tufts of fungal growth with spores develop on the under surface of leaves (Fig. 19). Later, the chlorotic spots become dark and brittle.
The affected plants remain stunted and may die if disease development continues because of favorable weather conditions.
Control: Avoid high plant densities and early sowing, which encourage greater disease development.
Spray with dithane M-45 to halt disease development.
Anthracnose
The disease is reported from Bangladesh, Canada, Ethiopia, Morocco and Syria. It is economically important only in Canada.
The disease is caused by Colletotrichum truncatum. The fungus is seed borne. The disease is favored by humidity and high temperatures (25-30°C).
Symptoms: Irregularly shaped, light brown lesions first appear on the base of the stem (Fig. 20) and then progress to the upper plant parts including leaves and pods.
Lesions on leaves and pods are few and circular with 1-2 acervuli in the middle of each lesion. Many lesions appear because of a rapid disease spread toward the end of the crop season. The affected plants topple on the ground.
Control: Seed treatment with fungicides such as benomyl (0.2-0.3%) or thiabendazole (0.1%) provides complete control of the seed-borne fungus.
Fungicide sprays of chlorothalonil or benomyl (I 2 L/ha) provide effective control of the disease.
|
|
