Transmission and biology of sunflower downy mildew

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Abstract
Sunflower seedlings grown in soil naturally infested with Plasmopara halstedii may not show symptoms of downy mildew, although the pathogen can be demonstrated in the tissues. Sporulation can be induced by placing infected seedlings in a saturated atmosphere for 12 hours. Inoculating seeds with a suspension of zoosporangia gave high percentage infection in about 14 days. Damping-off induced by P. halstedii, not previously attributed to this pathogen, occurred on sunflower seedlings in some inoculation experiments. Spore concentration was not critical for infection in the ranges tested, from 150 000 to 100 zoosporangia per milliliter. Zoosporangia stored on infected leaves at −20 °C germinated about 75% after 3 weeks, and about 10% after 14 weeks; none germinated after 24 weeks storage. Zoosporangia added to untreated soil remained infective for only 7 days; some remained infective in pasteurized and sterilized soils for 14 days. Zoosporangia did not germinate in distilled or tap water; germination occurred in 0.5% sucrose solution, but was better in 1 or 2% sucrose.

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