Abstract
A description is provided for Sclerospora sorghi[Peronosclerospora sorghi]. Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: Sorghum bicolor, Sorghum sudanense, Sorghum almum, Sorghum halepense, maize, teosinte and, apparently, Heteropogon contortus. DISEASE: Sorghum downy mildew (SDM); both systemic and local infections occur in sorghums, in maize usually only systemic. In systemic infections, chlorotic laciniate areas extend from bases of leaf blades (half-leaf symptoms), with successive leaves displaying greater or complete chlorotic involvement. Sporulation (white down) occurs on dewy nights; conidia, actively ejected, germinate and infect immediately, inducing local lesions on sorghum leaves but seldom on maize. Resting spores form in systemically-infected plants in immense numbers within chlorotic areas (less, or sometimes not at all in maize, according to geographic region and variety) and leaves of sorghum, but not maize, later shred. Systemically infected plants tend to be dwarfed and, on many cultivars, produce few or sterile inforescences. There may be late expression of chlorotic symptoms in some plants inoculated at seedling stage. Local lesions are elongate, yellowish, then purple, with white down, finally necrotic. GEOGRAPHICAL DISTRIBUTION: Widespread (CMI Map 179 cd. 2, 1966, with some additions and deletions). Africa: Nigeria, Egypt, Sudan, Kenya, (?) Congo, Uganda, Tanzania, Rhodesia, Zambia, South Africa. Asia: Israel, (?) Iran, Pakistan, India, Thailand, Nepal, China. N. America: USA, Mexico, Honduras. S. America: Venezuela, Brazil, Peru, Argentina. TRANSMISSION: Primary infection (systemic) in USA, Israel and part of India is by resting spores (oospores) which may remain viable in soil for a number of years. Conidial-induced systemic infection in all lands occurs on plants up to 4 weeks of age. and in Thailand is apparently the principal or only mode. With oospore infection, the frst leaf is never chlorotic: with early inoculation by conidia, the frst leaf may be infected and seedlings may die. Seed-transmitted infection has been proven (49, 744), mostly by adhering oospores, but when internal hyphae are present, no transmission occurs if seed is dried before sowing (52, 1114). Local lesions are induced only by conidial inoculation. Sowing in cold soil (less than 20°C) prevents infection by oospores (50, 1765). Conidia are normally viable only a few hours after sporulation which occurs after midnight to before dawn at 18-28°C (opt. 21°C) (15-30°C with opt. 22-25°C or 24-26°C in Thailand) and transmission of disease by air-borne conidia apparently is restricted to nearby fields. Opt. for conidial infection is 21-24°C. Resting spores were reported to retain infectivity after passage through digestive tract of cattle.
