Infection studies with watermelon wilt caused by Fusarium niveum EFS.

Watermelon wilt, caused by Fusarium niveum, is a serious disease in Iowa and in many other sections of the United States. The present Iowa acreage is less than 10 percent of the acreage before wilt became a factor. Three heretofore undescribed seedling symptoms are described. These are: seedling rot, damping off and stunting. The organism may cause lesions on any of the roots. These are at first water-logged and become discolored in advanced stages. The lesions vary in length from a trace to 28 centimeters. Infection may be induced thru the use of infested soil, by means of spore suspensions of the organism injected with a hypodermic needle into the hypocotyl and by the insertion of the mycelium into wounds. It is probable that the organism enters the host thru root hairs, root injuries and thru the epidermis of the hypocotyl. Fusarium niveum has been recovered from primary, secondary and tertiary roots, from stems, petioles, leaves, peduncles, melon flesh and seeds of infected plants. The organism has also been recovered from and observed in the vascular tissues of what appeared to be normal, healthy plants. The organism flourishes at high temperatures. Its optimum on potato dextrose falls between 24 and 32°C., the minimum below 12°C. and the maximum above 35 °C. The organism flourishes on acid media, the optimum falls between pH 4.6 and 6.0 and the minimum below 3.0. With proper technique, seedling susceptibility in the greenhouse gives a good index of the susceptibility to be expected in the field