Individually, green melon aphids (Aphis gossypi) and anthracnose (Colletotricum lagenarium) can cause serious economic damage to watermelons by reducing stands and marketable yields. Greenhouse-grown watermelon seedlings at the third true leaf stage were infected with anthracnose (106 spores/mL) and/or infested with 30 aphids per plant. At the 5th leaf stage (about 7 days after inoculation/infestation), leaf disks were harvested from plants and indicators of stress measured. Peroxidase activity increased from 0.03 to 0.28 absorbance units/mg protein-minute in leaves with anthracnose. When plants were infested with aphids after anthracnose inoculation, peroxidase activity was 0.40 absorbance units/mg protein-minute. Plants having both aphids and anthracnose had more anthracnose lesions when leaves were infested with aphids prior to anthracnose inoculation. The presence of aphids and/or anthracnose stimulated 1-aminocyclopropane-1-caroxylic acid (ACC) oxidase activity from 28 to 44 nL/g-h, indicating enhanced ethylene production. However, aphids had to be present on plants at least 5 days before ACC oxidase activity was stimulated above control levels. Aphids combined with anthracnose failed to elevate ACC oxidase levels higher than either aphids or anthracnose alone. Both peroxidase activity and ACC oxidase activity in watermelon plants increased with anthracnose infection. Thus, watermelon plants stressed by aphids and anthracnose responded differently from plants stressed individually by aphids or anthracnose.
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