Cause Fusarium oxysporum f. sp. niveum, a fungus that may survive several years in soil as durable spores (chlamydospores) or in association with plant debris. Seed can also transmit Fusarium wilt, usually by contamination of the seed coat. The disease is favored by warm, sandy soils like those in watermelon-growing areas east of the Cascade Range. There is specialization within this fungal species towards various plant hosts affected. These formae speciales of Fusarium oxysporum, which incite Fusarium wilt on cucurbits are as follows: f. sp. cucumerinum (cucumber), f. sp. melonis (muskmelon), and f. sp. niveum (watermelon). No formae speciales of Fusarium oxysporum has been reported for squash; however, some varieties of summer squash can be affected by F. oxysporum f. sp. niveum. There are also races within the formae speciales; at least three watermelon races, four melon races, and three cucumber races have been reported.
Symptoms The fungus can attack a susceptible plant at any stage of growth. Infection of the hypocotyl of young plants can result in pre- or post-emergence damping-off. Infection of older plants can cause yellowing (starting in the older leaves), stunting, or wilting, and once wilting occurs, death may result in 3 to 5 days. Diseased plants wilt but partly recover at night in warm weather. Wilting also may coincide with fruit maturity. The fungus affects the vascular system and infected plants may not show noticeable symptoms until they begin bearing fruit. Browning of the vascular system is usually evident in the lower stem, crown, or tap root. On the dying vine's stem, a pinkish white cottony growth can appear near ground level. Root rot is not observed.
- 'Klondike Blue Ribbon' has partial resistance under growing conditions in the Hermiston and Boardman, OR areas. Many other Fusarium-resistant varieties are available but have not been evaluated in the Pacific Northwest.
- Plant clean, quality seed of resistant varieties.
- Rotating 8 to 10 years to nonsusceptible grass and grain crops may help some.
Chemical control Using resistant varieties and fumigating soil has been effective in the Hermiston, OR area.
- Metam sodium (Vapam HL) at various rates. See label for details. Restricted-use pesticide.
- Regalia at 1 to 2 quarts/A as an in-furrow treatment. Does not benefit from the addition of an adjuvant. 4-hr reentry. O
- Actinovate AG at 3 to 12 oz/A as a soil drench at planting. 1-hr reentry. O
- Actinovate Lawn & Garden at 1/2 to 1 teaspoon/gal water. H O
- Cease at 4 to 8 quarts in 100 gal water applied to soil at 1 pint/sq ft for each 3-inches of soil depth on 21- to 28-day intervals. For greenhouse plants only. 4-hr reentry. O
- Double Nickel LC at 0.5 to 4.5 pints/A for soil application on 14- to 28-day intervals. Can be applied the day of harvest. 4-hr reentry. O
- Prestop as a soil drench or incorporation into growing medium. 4-hr reentry. O
- Rootshield Home and Garden at 1 to 3 Tbsp/gal water as a soil drench at planting. H O
- Rootshield WP at 3 to 5 oz/100 gal water for greenhouse soil drench and field chemigation or at 16 to 32 oz/A as an in-furrow spray. 0-hr reentry. O
- Stargus at 3 to 4 quarts/A as a soil drench on 10- to 21-day intervals. Preharvest interval is 0 days. 4-hr reentry. O