Influence of Acidity Level in Simulated Rain on Disease Progress and Sporangial Germination, Infection Efficiency, Lesion Expansion, and Sporulation in the Potato Late Blight System

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Abstract
The progress of late blight caused by Phytophthora infestans race 1.5 on potato (Solanum tuberosum ''Kennebec'') was monitored in field studies in which ambient rainfall was excluded and simulated rain events at pH 2.8, 5.6, 4.2, 4.8, or 5.6 were applied three times per week in 1984 and twice per week in 1985. There were significant quadratic and cubic effects of acidity of simulated rain on disease progress (p = 0.024) in 1985, but no significant effects in 1984. Disease increased most rapidly at pH 4.8 of simulated rain in 1985. Maximum disease severity in each year was approximately 15%. In laboratory studies, direct and indirect germination of sporangia of two isolates (race 1,5 and race 1,3,4,5) were almost completely inhibited in simulated rain solution at pH 2.4. Maximum direct germination occurred at pH 3.0, and maximum indirect germination occurred at pH 5.6 and 3.6 for isolates of race 1,5 and race 1,3,4,5, respectively. Infection efficiencies of sporangia borne in simulated rain solutions of pH 2.4, 3.0, 3.6, 4.2, and 5.6 were similar if misting of plants with deionized water (pH .apprx. 5.6) commenced immediately or 4 hr after inoculation; however, when misting was delayed until 8 or 16 hr after inoculation, significant numbers of infection occurred only with pH 3.6, 4.2, and 5.6 treatments. Infection occurred only at pH 3.2 and above when sporangia were applied in simulated rain at pH 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, or 3.6, and misting was delayed until 8 hr after inoculation. In the greenhouse, final size of late blight lesions did not differ among simulated rain treatment at pH .apprx. 3.0, 4.2, or 5.6; however, number of sporangia produced per square centimeter of lesion was lower at pH 3.0 than at pH 4.2 or 5.6. Although laboratory and greenhouse studies indicate that highly acidic rain could potentially reduce severity of late blight of potato, this potential was not realized in the field studies. Duration, frequency, and extent of exposure to highly acidic rain in the field may have been insufficient in our studies and may be insufficient in the ambient environment to produce discernible effects on severity of late blight of potato.