Fusarium wilt is a serious problem in muskmelons. It may also occasionally occur on watermelons and cucumbers. Most other cucurbits have good tolerance to this disease. All cucurbits can be symptom-less carriers.
Identification Early-season Fusarium wilt infections result in the damping-off of seedlings and transplants. Later infections cause the foliage to turn grey-green and wilt, followed by a generalized yellowing and defoliation. Initial stages are often mistaken for water stress and the disease remains unidentified until large portions of the field collapse and die.
Symptoms usually first appear on the crown leaves, progressing outwards along the vines. The leaf symptoms occasionally appear only on one side of the vine, while the other leaves remain green and healthy. The vascular tissue of infected crowns turns reddish-brown. Long, linear-shaped, brown stem lesions may form on one side of the vine, near the crown. Under certain conditions a gummy red exudate will ooze from these stem lesions.
Muskmelons from infected fields may develop raspberry-pink fungal growths on the underside of the fruit. This mould may also enter the fruit through the blossom scar, developing inside the melon cavity.
Often Confused With Water stress
Gummy stem blight
Biology Fusarium wilt is primarily spread via infested soil. While there may be limited seed transmission, it is quite rare. Fusarium spores may survive in the soil for several years. There are several different races of Fusarium known to infect muskmelon. Race 2 is considered to be the most prevalent race in Canada.
Muskmelon crops grown in slightly acidic soils (pH 5.0- 5.5) may be more prone to Fusarium wilt. A heavy fruit load, or excess soil nitrogen levels (particularly Ammonium-N) may further aggravate Fusarium infections. The initial infection occurs through the root. Fusarium may penetrate through the growing root tip, or though wounds caused by cucumber beetle larva or other root insects.
Period of Activity This disease may attack the plant at any stage, although outbreaks often follow periods of crop stress, including hot, dry weather conditions. Soil temperatures of 18- 25°C (64- 77°F) and 50- 65% relative humidity support rapid disease progression.
Scouting Notes While monitoring for other diseases, take note of any wilted plants. Record the number and location of any infected plants.
Thresholds There are no thresholds for this disease. Since the infection occurs on the roots, foliar fungicide applications will not prevent the spread of Fusarium wilt in the field.
Fusarium-resistant vine crop cultivars are available. As several different races of Fusarium exist, look for varieties with multiple resistance genes.
Fusarium wilt may survive in the soil on infected crop residue for several years following a melon crop. Follow a 3–4-year rotation away from all cucurbits and use only sterile, soil-less mix for cucurbit transplants.
Fusarium is a stress disease. Any condition that causes stress to the crop may result in increased levels of Fusarium.
Lime soil to a target pH of 6.0-6.5 and avoid over-fertilization, especially with fertilizers containing ammonium.
Maintain adequate soil moisture levels, especially during fruit-set and sizing.