Fusarium spp. causes plants to wilt. The fungus is soil-bourne but can sometimes be seen as a pink layer of fungal growth on red-brown sunken lesions at the base of wilting stems which bear yellowing leaves. Red-brown longitudinal streaks will be seen in the internal tissues if an affected stem is cut open near ground level. This fungus attacks the roots and rots the stem aswell as causing damping-off. The pathogen in wilt diseases is a xylem inhabitant, rather than the cortical pathogen as above, pluging the water conducting vessels. Small, dark, irregular spots initially appear on lower leaves, affected leaves suddenly become chlorotic. Wilt symptoms begin with an upward curling of leaf tips, leaves then dry to a yellow-tan colour and hang on plants without falling off. Stems also turn yellow-tan, with either whole plant or just one side wilting. Fusarium wilt is aerobic and a warm weather disease, infection via rain splash from debris, root rot penetration of young roots and in older roots an injury admits the fungus.
Verticillum spp. first turns leaves yellow along margins and between veins, then turn grey-brown. Lower leaves show symptoms first, slightly wilted plants may recover at night or after irrigation. These recoveries are transient as wilt becomes permanent. Dissection of stem reveals a brownish discolouration of xylem tissue. No chemical controls are known.
Remove all debris, avoid seedborne infection, encourage natural antagonists and reduce root wounding. Ca and K deficiencies and excess N (particularly nitrates)increase wilt disease. Trichoderma can be effective as a preventative.
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