Evaluation location: South Carolina, United States
Watermelon accessions were tested against a race 2
Sphaerotheca fuliginea (Schlechtend.: Fr.) Pollacci isolate to evaluate for resistance to powdery mildew disease. Growth room-grown seedlings were artifically inoculated with conidia from watermelon host leaves at 2-day intervals from the appearance of the first true leaf until test results were taken, when the second true leaf was fully expanded.
Plants were evaluated on a 1 to 9 scale of increasing disease severity where 1 = no evidence of infection, 2 = trace infection of cotyledons only, 3 = low infection of cotyledons only, 4 = moderate infection of cotyledons only, trace infection of leaves, or no infection of cotyledons, slight infection of leaves, 5 = severe infection of cotyledons, slight infection of leaves, 6 = severe infection of cotyledons, moderate leaf infection as evidenced by leaf deformity and or sporulation on leaves, 7 = severe infection with abundant sporulation on cotyledons and leaves, or on leaves only, 8 = severe infection of leaves with some necrosis of leaves, and 9 = plant dead due to powdery mildew.
Disease indices (DIs) were calculated as a weighted average for each of these entries. Plants having powdery mildew ratings of 1.0 to 3.0 were classed as resistant, 3.1 to 6.0 were classed as intermediate resistance, and 6.1 to 9.0 were classed as susceptible.