A Powdery Mildew on Cotton from Peru

Abstract

Powdery mildew is one of the most common diseases of cotton in Peru, occurring in all of the cotton-growing valleys of the coastal region where cotton is a major crop. In spite of its wide distribution and the heavy infections which frequently occur, how? ever, it is of minor economic importance. The disease is characterized by the appearance of scattered, cir? cular patches of mycelium, first on the lower and later on the upper surfaces of the leaves. The patches are usually numerous and enlarge until they coalesce to cover the entire leaf surface. When a large part of the leaf is covered by the mildew, it curls slightly, frequently turns yellowish, and may finally fall. Late in the season the heavy growth of the fungus on the leaves some? times gives the fields the appearance of being covered with a light fall of snow, giving rise to the local common name for the disease of manta blanca or white mantle. Although such heavy infections may cause partial defoliation, little damage results be? cause of the advanced stage of maturity of the plants. Field observations have shown that several North American varieties of cotton are more susceptible to the disease than either the native Peruvian cotton (Gossypium peruvianum) or the hybrid Tanguis, which is the principal commercial variety of the country. The heaviest infections have been noted on Acala. Pima, Delfos, Express and Super Seven are also susceptible. Because of the minor economic importance of the disease no control measures are suggested. The causal fungus has been identified as Erysiphe Malachrae Seaver after comparison of Peruvian collections with type material collected by Dr. N. L. Britton and F. S. Earle March 11, 1922 (Expl. of Porto Rico 6488), kindly furnished by Dr. Seaver. Both the imperfect and perfect stages usually occur in the field.