Vascular wilt diseases Fusarium oxysporum and Verticillium dahliae

These fungi belong to the Deuteromycota group of fungi.

Damage. These two organisms infect the xylem tissues of horticultural plants, causing the leaves to wilt in hot conditions, a symptom which can also be caused by other factors, e.g. lack of soil moisture (see wilt) and nematode infestation (see root knot nematode p230). The wilt diseases can be recognized by yellowing and eventual browning of the lower leaves (see Figure 15.17) and by brown staining of the xylem tissue when it is exposed with a knife. Verticillium may attack a wide range of plants, e.g. dahlia, strawberry, lilac, tomato and potato, so that rotation is not a feasible control measure. Fusarium oxysporum, however, exists in many

Fusarium Potato

Figure 15.16 Fusarium patch on turf. Note the area of dying turf. In or dr>' summer conditions as dormant mycelium in the earlier stages of the disease, distinct circular patches about 30 cm dead leaf matter or newly infected leaves. across are seen.

Control. Preventative control measures are important. Avoid high soil nitrogen levels in autumn, as this promotes lush, susceptible growth in autumn and winter. Avoid thatchy growth of the turf, as this encourages high humidity and thus favours the disease organism. The groundsman can drench preventative fungicide such as iprodione in autumn to slow down infection of the fungus. Summerapplied systemic fungicide such as thiophanate methyl is able during the actively growing period of the year to move within the plants and achieve curative control.

distinct forms, each specializing in crops in different plant families, e.g. tomato, cucumber, bean or carnation.

Life cycle and spread. Both organisms may live as saprophytes in the soil. Fusarium survives unfavourable conditions as thick-walled asexual spores, while Verticillium forms small sclerotia. Infection by both genera occurs through young roots or after nematode attack in older roots. The fungal hyphae enter the root xylem tissue and then move up the stem, sometimes reaching the flowers and seeds. The diseases are spread by water-borne asexual spores. The two fungi have different temperature preferences. Verticillium more commonly attacks in springtime, having an optimum infection temperature of 20°C, while Fusarium is more common in summer, with an optimum temperature of 28°C.

Control. Control is often necessary in greenhouse crops. Infected crop residues should be carefully removed from the soil at the end of the growing season. The amateur gardener or professional grower may choose to use peat bags instead of soil. The professional may use partial soil sterilization by steam, or a chemical sterilant such as metam-sodium.

In unsterilized soils, professional growers may use resistant rootstocks, e.g. in tomatoes, which are grafted onto scions of commercial cultivars. Rotation may be employed against a Fusarium oxysporum attack, as different forms attack different crops. Careful removal of infected and surrounding plants, e.g. in carnations, may slow down the progress of the diseases, especially if the soil area is drenched with a systemic chemical such as carbendazin, which reduces the infection in adjacent plants.

Bacteria

These minute organisms (see Figure 15.2) measure about 0.001 mm and occur as single cells that divide rapidly. They are important in the conversion of soil organic matter (see Chapter 18), but may, in a few parasitic species, cause serious damage or losses to horticultural plants. Some details of their classification are given in Chapter 4.

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