Cause Fusarium wilt is caused by the fungus, Fusarium oxysporum f. sp. cucumerinum, which may persist for long periods in the soil as durable spores (chlamydospores) or in association with plant debris. Seed can also transmit Fusarium wilt, usually by contamination of the seed coat. Melon seeds may have an internal infection just under the seed coat.
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. benincasae (wax gourd), f. sp. cucumerinum (cucumber), f. sp. lagenariae (calabash gourd), f. sp. luffae (vegetable sponge), f. sp. melonis (muskmelon), f. sp. momordicae (bitter melon), 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, the wilt strain, which affects watermelon. 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. Affected melon plants may develop a lesion on the lower stem. 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. After the plant dies, a white mycelium mat may form on external plant surfaces. Root rot is not observed.
- Plant clean, quality seed of resistant varieties. Because there are races of wilt forms, it is necessary to know, which races are present before choosing a resistance variety.
- Plant on land not previously cropped with the cucurbit species.
- Liming applications to bring the soil pH to 6.5 to 7.0 can reduce disease.
- Do not move soil from infested fields.
- Not replanting to the same cucurbit species for 5 to 7 years can help manage watermelon wilt but is consider ineffective for melon and cucumber wilt.
Biological control Efficacy in the Pacific Northwest is unknown.
- Actinovate AG at 3 to 12 oz/A as soil drench at planting. 1-hr reentry. O
- Actinovate Lawn & Garden at 0.5 to 1 teaspoon/gal water. H O
- Double Nickel LC at 0.5 to 4.5 pints/A as a soil drench at planting. 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
- Taegro ECO at 2.6 to 5.2 oz/A applied as a drench, in-furrow at planting, or through drip irrigation. 24-hr reentry. O