Identification of Fusarium Wilt and Bacterial Wilt in Cucurbits and Vegetables and Comprehensive Control Measures

According to surveys, Fusarium wilt and bacterial wilt occur in many types of cucurbits and many kinds of vegetables in our city. For example, Fusarium wilt can occur in watermelon, muskmelon, cucumber, winter melon, chili pepper, tomato, and potato; chili pepper, tomato, potato, eggplant, and some leguminous vegetables are affected by both Fusarium wilt and bacterial wilt. These diseases often cause 30-50% losses in cucurbits and vegetables, and can even lead to total crop failure. Therefore, correctly distinguishing and identifying these two diseases and adopting corresponding comprehensive control measures is particularly important.

1 Identification of the Two Diseases

These two diseases often occur simultaneously on cucurbits and vegetables, and their external symptoms are extremely similar in some respects. The most notable feature common to both diseases is that they are vascular bundle diseases, manifesting as plant wilting and blighting. In the early stage of onset, both present as drooping and wilting of leaves and shoot tips, especially obvious at noon or under strong light, while recovering to normal in the early morning and evening; in the mid stage of onset, when diseased plants are pulled up and the stems are dissected, the vascular bundles have already turned brown or dark brown; in the late stage of onset, the diseased plants wither and die. Although the symptoms of the two diseases are very similar, they can be distinguished as long as field diagnosis is carefully carried out and the entire disease development process is monitored.

1.1 The pathogen species are different. Fusarium wilt is a fungal disease caused by Fusarium oxysporum of the genus Fusarium, subphylum Deuteromycotina, while bacterial wilt is a bacterial disease caused by the rod-shaped bacterium Ralstonia solanacearum.
1.2 The speed of plant wilting differs. Bacterial wilt develops rapidly; from the onset of symptoms, it takes only 4-6 d for the plant to wilt and die, presenting as acute wilting of the whole plant, while Fusarium wilt diseased plants begin with leaf drooping and wilting, then blight until death, requiring 12-15 d; the speed and time of wilting is 8-9 d slower than the former.

1.3 The site of leaf wilting on diseased plants differs. Fusarium wilt diseased plants start from the lower leaves, wilting sequentially from bottom to top. Leaf color gradually changes from green to pale, from yellow to withered yellow, and then to brown. Leaves basically do not fall off, and the entire plant dies. For example, muskmelon Fusarium wilt can occur throughout the entire growth period, with the heaviest incidence from flowering to fruit-setting stage. When seedlings are affected, roots are few, leaves curl, and then the whole plant collapses and dies. In adult plants during flowering and fruit-setting, leaves wilt sequentially from bottom to top; at the initial stage of onset, they wilt during the day and recover to normal in the morning and at night; after 10 d, leaves become withered and dead, and the plant is easily pulled up. Another example is watermelon Fusarium wilt, which mostly occurs after fruit set in adult plants. Diseased plants grow slowly, lower leaves first wither and turn yellow, progressively developing upward. At first, they wilt during the day and recover in the morning and evening; after several days, the whole plant wilts and dies. Bacterial wilt diseased plants differ from Fusarium wilt in that the young leaves at the upper tips, tender shoots, and newly unfolded tender leaves wilt. When wilting, the leaves are still green. In the early stage of onset, they can still recover to normal in the morning and evening each day. Subsequently, leaves in other parts acutely wilt and droop, rapidly wilting. Before the leaves turn yellow and change color, the whole plant quickly loses water, becomes green-wilted and withers. For example, bacterial wilt of chili pepper, tomato, and potato all manifest as wilting of green leaves and green branches.

1.4 The symptoms and signs manifested by diseased plants differ. On Fusarium wilt diseased plants when moist, yellow-white or pink mold-like substances can be seen at the base of the stem (vine or tendril); these are the conidiophores and conidia of the fungus. For example, on the base of stems of cucurbit Fusarium wilt, brown striped spots or longitudinal cracking of the epidermal layer commonly occur. When moist, resinous gum exudes, the vascular bundles of the stem turn brown, the vine becomes water-soaked and rots, and white cornmeal-like or pink mold-like substances appear. However, bacterial wilt diseased plants only show symptoms on the external stems, leaves, branches, and other parts, without presenting signs (pathogens). Only after cross-sectioning the diseased stem and squeezing the discolored vascular bundles by hand does white bacterial fluid overflow; this is its pathogenic bacterium. If the cross-sectioned diseased stem is immersed in a glass bottle containing light salt water, milky-white mist-like substances can also be seen spraying out from the cut surface.

2 Comprehensive Control Measures

Fusarium wilt and bacterial wilt of cucurbits and vegetables are two stubborn diseases that are very difficult to cure. They can spread through soil, seeds, fertilizers, manure, water flow, and crop wounds caused during agricultural operations carrying the bacteria. The pathogens invade from root wounds and root hair tips, secrete toxins in the crop's vascular conduits, clog the conduits, affect the normal transport of water and nutrients, and cause crop wilting and death. The conditions for disease onset are high temperature, high humidity (20-25°C, relative humidity above 80% is suitable for onset), continuous cropping, excessive application of nitrogen fertilizer, poor drainage and irrigation, undecomposed fertilizer, and deficiency of phosphorus and potassium, all of which can lead to the occurrence of Fusarium wilt and bacterial wilt.
2.1 Rational crop rotation. The pathogens of both Fusarium wilt and bacterial wilt are “soil-inhabiting fungi”. Fusarium wilt pathogens overwinter in the soil as mycelium, thick-walled spores, or sclerotia, and can live saprophytically in diseased residues for 5-7 years; bacterial wilt pathogens can also overwinter in the soil with diseased residues, surviving from 14 months to 7 years. Therefore, as long as these two pathogens remain in the soil, once infected in 1 year, the disease will occur every year. Therefore, a 2-5 year rotation to different crops should be implemented on land where the disease has occurred, or rice-upland crop rotation has an even better effect. At the same time, before planting, diseased residues must be thoroughly removed and burned, diseased plants should not be used for composting, undecomposed organic fertilizer should not be applied, agricultural operations should minimize wounds on cucurbit and vegetable plants, drainage and irrigation conditions should be improved, and the source of bacteria should be avoided from multiple aspects.