Studies On The Powdery Mildew Disease Of Galphimia Glauca Ca V. Caused By Leveillula Taurica (Lev.) Arn. And The Effect Of Water Stress Imposed On The Host Plant On The Fungus.

Abstract

ABSTRACT The morphology of the isolate of Leveillula taurica parasitizing Galphimia glauca at Legon and the effect of water stress imposed on the host plant and on the fungus and infection rate were studied.

L. taurica forms dimorphic conidia, conical or primary conidia and cylindrical or secondary conidia. The cylindrical conidia of this isolate were longer than the conical conidia. The two types of conidia were, however, physiologically identical.

The conidia germinated from zero to 100% RH. Percentage germination in 24 hours of conidia from plants watered daily and with highest Relative Water Content (R.W.C.) was 20.8%. Percentage germination then decreased with decreasing atmospheric relative humidity. Conidia germination was better in light of 76 lux and the germ tubes produced were longer than in dark. Furthermore, conidia formed in light had greater vigour than those formed in dark. Greater germination was stimulated on surfaces of host leaves watered daily. Percentage of conidia obtained from plants watered daily which germinated in 24 hours was 40.0,38.4 and 22.4 percent when inoculated onto host leaves of 80.6, 74.4 and 66.7 percent RW.C., respectively. Water stress imposed on the host plant affected the morphology of the fungus and germination capacity of the conidia. Of the four levels of R.W.C. (80.6, 78.4, 74.4 and 66.7%) maintained in the host leaves, diameter of haustoria, length of unbranched conidiaphores, branching, size of the conidia, level of water content of the conidia, and the germination capacity of the conidia decreased with increasing water stress. Both the R.W.C. of the leaf on which the conidia were formed and the RW.C. of the leaf inoculated with the conidia influenced infection rate. The best combination was a high R.W.C. of source plant and a high R.W.C. of the inoculated plant.

Infected leaves became chlorotic with 'green islands' and were shed prematurely. The flowers were not infected and no mycelium was detected in the stem. It was concluded that the G. glauca isolate was different in some aspects from the Capsicum sp. isolate. Also, because weaker conidia were produced on leaves with greater water stress, infection rate could be reduced by imposing water stress on the plants to a level, which may not unduly harm the plants, but result in the formation of less effective infection propagules.