Recently, Fusarium crown rot was diagnosed on a commercial planting of summer squash. Different Fusarium species can attack watermelons, cantaloupes, cucumbers, summer squash, winter squash, and pumpkins. This is a very difficult disease to control with rotation and resistant varieties where available being the best options. The following is some information on this fungus.
Fusarium wilt of melon (cantaloupes and muskmelons) is caused by a seed- and soilborne fungus that is specific to melon (Fusarium oxysporum f. sp. melonis). On young seedlings, a hypocotyl rot and damping-off may occur. In older plants, there is marginal yellowing progressing to a general yellowing of the older leaves, and wilting of one or more runners. In some cases, sudden collapse occurs without any yellowing of the foliage. On stems near the crown of the plant, a linear, necrotic lesion may develop, extending up the plant and usually on one side of the vine. One runner on a plant may wilt and collapse, with the rest of the runners remaining healthy. A gummy, red exudate may ooze from these lesions, but this may also be caused by gummy stem blight and insect injury. Vascular discoloration should be evident and is very diagnostic. Mature plants often wilt severely (collapse) late in the season because of the fruit load stress. Disease severity is maximum at soil temperatures of 64-77°F and declines dramatically above 86°F. At high soil temperatures, plants become infected but may not wilt; rather they develop severe stunting. As with most Fusarium wilts, the following apply: 1) low soil moisture favors the pathogen and accentuates the wilting symptom; 2) high nitrogen, especially NH4-nitrogen, and light, sandy, slightly acidic soils (pH 5-5.5) favor disease development; and 3) liming the soil to increase the pH to 6.5-7.0 decreases wilt severity. Crop rotation is generally not totally effective because chlamydospores survive so long in the soil and the pathogen can survive in or on the roots of symptomless carrier plants. Soil fumigation with a broad-spectrum biocide provides good initial control, but recolonization of the soil occurs very quickly. Liming the soil to pH 6.0-7.0, as well as reducing nitrogen levels in the soil, significantly reduces wilt. The most effective and practical means of controlling Fusarium wilt of melon is through the use of resistant varieties. Work to develop resistant varieties began as soon as the disease was described, and now resistance genes conferring resistance to races 0, 1, and 2 have been incorporated into U.S. commercial varieties and hybrids. One such hybrid that performs well in Delaware is the variety Athena.
Fusarium crown and foot rot of squash and pumpkin is caused by Fusarium solani f. sp. cucurbitae. The pathogen exhibits host specificity for all cucurbits. Only certain squashes and pumpkins seem to be affected. The organism is also thought to exist as races. While its distribution in the United States is limited, it still remains a concern for cucurbit growers. We have seen the crown and foot rot phase of disease sporadically in Delaware and the fruit rot phase has been found in pumpkins in the state. Symptoms. The first symptom usually noticed in the field is wilting of the leaves. Within several days, the entire plant may wilt and die. If the soil is removed from around the base of the plant, a very distinct necrotic rot of the crown and up per portion of the taproot is evident. The rot develops first as a light-colored, water-soaked area which becomes progressively darker. It begins in the cortex of the root, causes cortex tissue to slough off, and eventually destroys all of the tissue except the fibrous vascular strands. Infected plants break off easily about one inch below the soil line. The fungus generally is limited to the crown area of the plant. The main and lower portions of the taproot are not affected, except under extremely wet conditions. Likewise, the stem is not affected, except for the lower inch immediately above the soil line, as seen in experimentally inoculated pumpkin seedlings. Plants showing symptoms develop numerous sporodochia and macroconidia (spores) giving the mycelia a white to pink color on the stem near the ground surface. Fruits are attacked at the fruit-soil interface; the severity of the fruit rot depends on soil moisture and the stage of rind maturity at the time of infection. Because the fungus survives in the soil for only two to three years, a four-year rotation is usually adequate for disease control. Planting fungicide-treated seed also reduces disease initiated from infected seed.
Fusarium wilt of watermelon is caused by the fungus Fusarium oxysporum f. sp. niveum. In the USA, we have two types or “races,” of Fusarium wilt of watermelon. Race 1 is the most common type. Plant breeders have developed many watermelon varieties that are resistant to race 1. Some varieties, like Jubilee; have a low level of resistance, others have a medium level, like Charleston Gray; and some, like Calhoun Gray, have a high level of resistance. This was all well and good for the past 35 years or so. As hybrids with resistance to race 1 were grown more widely and more often, the Fusarium fungus began to mutate. Some mutants turned out to be “super bugs,” able to attack these resistant varieties. These new strains are called race 2. Resistance to race 1 will not prevent melons from getting Fusarium wilt caused by race 2. Race 2 is worse than race 1, because there is no resistance to race 2. So far, race 2 has been found in Texas, Oklahoma, Florida, Maryland, Delaware, and Georgia, but not South Carolina. Most, but not all, diploid (seeded) watermelon varieties on the market today have resistance to race 1. Examples of varieties that have been consistently resistant in field plots in North Carolina, Georgia, Florida, and Maryland are: Crimson Sweet, Jubilee II (not Jubilee), Royal Star, Royal Sweet, Royal Majesty, and Sangria. All triploid (seedless) cultivars are susceptible to race 1 except Seedless Sangria. All commercial watermelon varieties are susceptible to race 2, which is why this disease remains a threat to the watermelon industry. Won’t fumigation control Fusarium wilt? Yes and no. Yes, fumigation will knock Fusarium back, especially if it was not too widespread or severe the previous year. But Fusarium can survive in soil deeper than any fumigant can reach. Of course, the pathogen also survives in nonfumigated soil in alleys between fumigated beds. Fusarium can wait deep in soil (at least 2 feet) or in the alleys until a watermelon root grows that deep or that far, then germinate and infect. So, fumigation can, and often does, help, but it will not eliminate (eradicate) Fusarium altogether. A new promising management tool studied in Maryland is hairy vetch as a winter cover crop. When hairy vetch is disked into soil in the spring, the residue produces ammonia, which is toxic to Fusarium. Like fumigation, hairy vetch will not eliminate Fusarium, but it will knock it back. ‘Cahaba White’ hairy vetch is the recommended variety because it is resistant to root-knot nematode. The vetch can be sprayed with herbicide before disking in the spring.
Information from Cornell University and Clemson University.