Abstract
A description is provided for Sclerospora graminicola. Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: The two hosts on which this pathogen causes diseases of major economic importance are Pennisetum typhoides[Pennisetum glaucum] (syn. P. americanum) and Setaria italica. Also recorded on Echinochloa crusgalli, E. crusgalli var. fumentacea, Eleusine indica, Panicum miliaceum, Pennisetum leonis, Saccharum of ficinarum (by inoculation only), Setaria lutescens, S. magna, S. verticillata, S. viridis and Zea mexicana. The disease is very rare on Zea mays with only two confirmed reports, Melhus & Bliss (1928) in the USA and Kenneth (1966) in Israel. DISEASE: Graminicola downy mildew; green ear of pearl millet (Pennisetum typhoides[Pennisetum glaucum]). A biotrophic plant pathogen which invades and colonizes the growing points of young graminaceous plants causing systemic disease. The first leaf to show symptoms is yellowed in the basal portion with a distinct margin between the basal colonized portion and the non colonized distal portion. Leaves formed later show increasing amounts of disease until the entire leaf shows symptoms. Under suitable conditions sporangia form in great profusion on the under surface of the diseased leaf (and, when conditions are favourable, also on the upper surface) forming a conspicuous and characteristic white 'down'. Occasionally discrete local lesions have been observed on otherwise healthy leaves in highly susceptible cultivars in W. Africa. The most distinctive appearance of the disease on pearl millet is, however, the transformation of the inflorescences to vegetative structures with various leaf-like protrusions which vary greatly in size and number from very few on an almost normal inflorescence to complete replacement of the inflorescence by small leafy shoots. As diseased organs mature they become necrotic and often contain oospores within the tissue. In pearl millet the leaves containing oospores do not shred. The areas containing oospores are a deep chocolate brown and usually appear as long stripes down the leaf. On Setaria the symptoms are similar to those observed on pearl millet except that shredding of the leaf tissue containing oospores occurs. In the phase of the disease described above the height of the infected plants differs little from that of healthy plants. Another and less frequent reaction is that the diseased plants are severely stunted, show a yellow mottle, with non-infected parts becoming a much darker green than in healthy plants; few sporangia are produced and no green ears for the plants generally do not head. This reaction is a characteristic response of certain host genotypes. GEOGRAPHICAL DISTRIBUTION: CMI Map 431, ed. 2, 1979. Note that the pearl millet pathotype has not been reported from the Americas. TRANSMISSION: Initial infection is by oospores which may remain viable for up to 10 years (Nene & Singh, 1976). Later infection comes from sporangia developing on early diseased leaves and spread by wind and rain to newly developed tillers which are produced throughout the growth of the plant (Singh & Williams, 1980). Seed transmission occurs from oospores carried with, and on, seed and there are conflicting reports of transmission from mycelium carried within seed (Williams, 1980).
Inheritance of Complete Resistance to Pearl Millet Downy Mildew
