Inoculation of pearl millet with the downy mildew pathogen, Sclerospora graminicola: chilling inoculum to delay zoospore release and avoid spray damage to zoospores

Inoculation of plants by spraying with a suspension of zoospores (released from sporangia of Sclerospora graminicola) induced immediate encystment of the zoospores and drastically reduced their ability to cause disease. The i ncidence of infection in spray-inoculated pearl millet seedlings was reduce d from 90% when sporangia were sprayed, to less than 30% when zoospores wer e sprayed. Rapid encystment was observed when zoospores were sprayed from b oth a hand-pumped sprayer and a compressed-gas sprayer and was probably cau sed by shearing forces. Chilling suspensions of sporangia prior to spraying delayed zoospore release and was an effective method for maintaining infec tion potential. Disease incidence was higher when sporangia were chilled to 0.2 degreesC rather than to 4 degreesC. Chilling resulted in some abnormal zoospore structures being released from sporangia when suspensions were re turned to 20 degreesC. The frequency of these structures increased with inc reasing chilling time. For seedlings sprayed with sporangia before zoospore release, a small reduction in disease incidence was observed when chilled inoculum was used, probably due to cold disruption of zoosporogenesis. For large-scale disease resistance screening, this reduction is outweighed by t he benefit of a uniform and adequately high disease pressure that can be ob tained over many hours using chilled spore suspensions.