Complete Abolition of High Inoculum Threshold of Two Mycoherbicides (Alternaria cassiaeandA. crassa) When Applied in Invert Emulsion

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Abstract
Fungal spore infectivity on leaves is a function of environmental factors (duration of dew point humidity, temperature), the defense mechanisms of the host plant, and fungal pathogenicity. The inoculum threshold of thousands of spores per square centimeter of leaf surface or tens to hundreds of spores per droplet has been expected and accepted. Evidence is presented that the high threshold concept does not hold with Alternaria cassiae and A. crassa, as one spore per 2-.mu.l droplet was sufficient to infect plants of Cassia obtusifolia and Datura stramonium, respectively, when the droplet was an invert emulsion containing a 1-.mu.l mixture of oils and waves on the outside and 1 .mu.l of water, sodium alginate, and conidia on the inside. The intensity of infection always was enhanced by the emulsion. Even a culture of A. cassiae that lost its infectivity, giving only a hypersensitive response when applied in water, became infective in the invert emulsion.