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Disease Management

Fusarium wilts are difficult to manage without durably resistant cultivars; however, there are a number of options that, while they may not be entirely effective alone, can help lessen the severity of the disease.

Disease-free transplants and seed. Exclusion of the pathogen is one of the best means of disease control. Therefore, transplants and seeds should be obtained from reliable sources and be disease-free. Transplants should be inspected for symptoms of Fusarium wilt (Figure 22) and discarded as appropriate. The reuse of transplant trays is not recommended since they may harbor low levels of FON and thus spread Fusarium wilt to new transplants. Limiting the spread of FON via transplants will help to prevent the introduction of the pathogen into new fields, as well as limit the introduction and dissemination of new races.
Host resistance. Genetic resistance to Fusarium wilt is the best and most economical method of control. However, there is no complete resistance to all races available in any commercial watermelon lines. Many diploid cultivars demonstrate excellent resistance to race 0 and race 1; however, this resistance may break down under high soil populations of the pathogen. There is no high level of resistance to race 2 currently available in commercial watermelon hybrids or cultivars. The advanced PI breeding line, PI-296341-FR, released a number of years ago has excellent resistance to all three races but incorporating the resistance into commercial types has proven difficult. There are reports of a few hybrids having some resistance or tolerance to race 2 but none that are commercially acceptable.
Crop rotations. Watermelons should never be grown in succession on the same land. Ideally, watermelon should be planted only every 5 to 7 years with non-susceptible plants grown in rotation. While this can help limit the build up of FON in the soil, chlamydospores are resilient and may remain viable in the soil for many years. Therefore, while successive cropping of watermelons can cause the pathogen to build up to high levels, crop rotation by itself is not likely to control the disease without additional management strategies.
Soil Fumigation. Moderate reductions in disease severity have been observed with commercial soil treatment products containing methyl bromide, chloropicrin and metam sodium. However, fumigation with these chemicals is seldom completely successful, primarily because of the resistant nature of the chlamydospores, roots that grow from the fumigated portion of the field into non-fumigated, infested portions of the field (below the fumigated zone or between fumigated rows), re-colonization and/or re-infestation of the soil by the pathogen, or because of improper application of the fumigants. In addition, many compounds once approved for soil treatment, including methyl bromide, are being phased out because of health and environmental concerns. Finally, if the pathogen is introduced into a previously fumigated field via infected seeds or transplants, the fumigation would have little effect on control of the disease. In some situations, soil fumigation may not be an economically viable option.
Soil solarization. Soil solarization is the use of clear plastic mulch placed over fallow soil during the hot months of the year in order to trap the energy from the sun and heat the soil to temperatures that are lethal to the pathogen. It has been used effectively in some locations around the world; however, it is costly and laborious. In addition, disposal of the used plastic is an environmental concern. Soil solarization can be used to lower populations of FON in soil sufficiently to delay the onset of wilt symptoms as well as in reducing disease incidence; however, it does not eliminate the disease and its effectiveness is limited to areas of the world where appropriate climatic conditions exist.
Grafting. The use of other cucurbit species not susceptible to FON (primarily Cucurbita spp. and Lagenaria spp.) as rootstocks for grafting watermelon (Citrullus lanatus) has been used successfully to control Fusarium wilt of watermelon. Grafting has been used most commonly in the Asian and Mediterranean basin areas. It has not been used in the United States primarily because it is labor-intensive and costly.
Biological control. Numerous studies have examined the potential of various types of biological control for Fusarium wilts in a number of crop plants, including watermelon and melons. While many demonstrate promise in laboratory and greenhouse trials, few have shown significant control at the field level. Examples of some of the approaches investigated are 1) the use of antibiotic-producing soil fungi and bacteria i.e., Gliocladium spp., Trichoderma spp. Pseudomonas spp.; 2) use of non pathogenic strains of F. oxysporum that compete with pathogenic forms for root colonization, i.e., F.o. strain 47; 3) the use of other formae speciales and races of F. oxysporum to induce resistance in plants to pathogenic forms; 4) natural or induced soil suppressiveness; and 5) the use of cover crops such as hairy vetch as soil amendments (“green manures”). While none of these give adequate control in the field, their importance as one more control strategy may increase as the soil fumigant methyl bromide is phased out of production and use.

With the exception of host resistance, it is likely that no one technique will result in the complete control of Fusarium wilt of watermelon. However, by combining several different options, adequate control may be achieved.

Figure 22

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