Every season in Coachella Valley, Calif., watermelon seedlings are transplanted into methyl bromide or metam sodium treated soil and covered with clear plastic mulch in January. Planting conditions are less than ideal. In the desert, there is always the danger of frost in January and February. Strong winds at that time of the year will constantly whip the plants around. This article will deal with a few soil-borne fungi that present potential disease problems for those plants that survived the early season challenges.

Sudden wilt

Several species of Pythium, including Pythium myriotylum, P. ultimum, and P. aphanidermatum, cause wilting symptoms, plant death and substantial crop loss of watermelons. Pythium spp. are water molds. Wet soils favor development of disease caused by any of these species. The optimum soil temperatures for P. myriotylum and P. aphanadermatum are from 90.8 to 99.8 degrees. Pythium ultimum is favored by much lower temperatures.

Decayed feeder roots are the first signs of the disease. As the disease progresses, brown water soaked lesions appear on the laterals and eventually the taproot is affected. Affected plants have reduced root capacity, and will wilt during the heat of the day. Plants may die, or they may recover during cooler periods when transpiration rates are lower, but the wilt will reoccur when temperatures and transpiration rates increase. Plant stress due to a heavy fruit load, or late season water deficit stress followed by irrigation often results in collapse of plants with affected roots.

Pythium spp. produce motile spores (zoospores) that can swim short distances and thick-walled resting spores (oospores) that are capable of surviving for two to 12 years in the soil. It is not unusual for the disease to spread down and across the rows. All watermelons are susceptible to one or more Pythium spp.

In the Imperial Valley, Pythium spp. have been detected in surface water used for irrigation. The fungus can be carried into the field with each irrigation.

Sudden wilt disease development is favored by saturated soil conditions, so avoid waterlogged soil conditions to prevent diseases caused by Pythium spp. Plant on raised beds to allow for maximum drainage, use alternate furrow irrigations, and avoid saturating beds to reduce the likelihood that this disease will occur.

In the Imperial Valley, soil solarization did not reduce disease incidence is a field study by Carl Bell and Frank Laemmlen.

Ridomil Gold EC (mefenoxam) is effective when applied before the disease is at advanced stages of development. It is more likely that an application will be successful in controlling this disease if it is applied when the earliest symptoms are present, or before symptoms appear.

Vine decline

Vine decline of watermelon, caused by Monosporascus cannonballus, is a common and costly problem in desert production areas. The disease first becomes obvious two to three weeks before harvest when the crown leaves begin to turn yellow and die. All of the vines within a field can collapse. Fruit on affected vines are usually smaller than normal, they have low sugar and often the fruit sunburn due to the lack of canopy cover. The number of feeder roots is reduced and the secondary roots will have tan to red-orange lesions with well-defined margins.

Late in the season or frequently after harvest, the fungus produces spores (ascospores) capable of surviving in the soil without a host for many years. These spores are produced within round black structures (perithecia), which protrude from the root tissue and can be seen with the naked eye. The perithecia are the diagnostic feature of this disease; however, these structures will only form on dead roots.

Plant collapse occurs due to the presence of the fungus on the roots and plant stress due to factors such as high air and soil temperatures, heavy fruit load, and water stress and insect infestations. When these conditions are present, the root system the plant produces blockages (tylosis) in the water conducting vessels of the plants, which lead to plant collapse.

If plant stress can be reduced, the likelihood of losing the crop to this disease would be reduced. In addition, avoid continually monocropping hosts of M. cannonballus, such as watermelon, cantaloupes and mixed melons season after season.

Fusarium wilt

Fusarium wilt of watermelons is caused by Fusarium oxysporum f. sp. niveum. Frequently the first Fusarium wilt symptoms noticed are wilting, yellowing and stunting in mature, fruit-bearing plants. Soon after these symptoms are seen, entire plants may collapse. The symptoms are a result of the plugging of the water conduction vessels (xylem) by materials produced as a result of the activity of this fungus.

Although infection may occur at any stage of plant development, the first striking symptoms are frequently expressed during fruit development due to increased water demand at this stage of development. Three races have been identified, but only Race 1 has been identified in California. Many race 1-resistant varieties are available, but extremely high levels of inoculum can overcome resistance. It is not unusual for a few affected plants to be present in a field even if the variety is resistant.

The fungus enters plants through roots and disease develops rapidly when soil temperatures are 64 to 77 degrees and symptom severity declines dramatically when soil temperatures exceed 86 degrees. In susceptible hosts, the fungus invades the xylem and produces spores (microconidia) that move through the vascular elements. In resistant hosts, the fungus is limited to the cortex or to only a few vessel elements.

The fungus can be seed-borne, or carried by water, on tools, machinery and the feet of field workers. F. oxysporum f. sp. niveum produces resting spores (chlamydospores) capable of surviving in the soil as a saprophyte for up to 16 years in the absence of watermelons; therefore, crop rotation is not an affective technique for management of Fusarium wilt.

Within a field, the disease generally spreads down and across rows as spores are splashed and blown from vine to vine. This fungus can survive in watermelon seeds for up to two years.

Fusarium wilt control options are limited to planting clean seed of wilt-resistant cultivars and if possible, planting on land without a watermelon history, or rotating to non-susceptible crops for five to seven years. Excessive rates of nitrogen favor the disease, so carefully manage the fertilizer program.

Methyl bromide fumigation or an application of metam sodium will reduce inoculum levels in the soil. However, F. oxysporum f. sp. niveum can colonize quickly, so chemical treatments are not recommended. Immediately after harvest, it is advisable to destroy the plants as the fungus will continue to propagate on them.