Fusarium wilt

Symptoms

The soil-borne fungus infects the root system and enters the vascular bundles, blocking the flow of water within the plant. The plants wilt, first on hot days and often on one side or from the lower leaves, then turn yellow and die despite moist soil. A cross-section of the stem reveals a characteristic brown discolouration of the vascular bundles. The disease intensifies in warmth — the pathogen develops best at high soil temperatures (about 25–30°C), which is why the first symptoms usually appear in midsummer. In the early phase the plants wilt during the hot midday and temporarily recover at night or after watering; over time the wilting becomes permanent. Yellowing progresses from the oldest, lower leaves upwards and is often distinctly one-sided — sometimes only half a leaf or one side of a shoot is affected, on the side along which the blocked vascular bundles run. The plants are stunted (growth is inhibited), and in seedlings fusarium wilt appears as damping-off and toppling of the transplants. Brown discolouration of the vascular bundles is the most reliable diagnostic feature — in fusarium wilt it is usually intense and reaches deep into the stem and the roots. Under moist conditions, especially at the base of the stem or on the cut surface, a delicate pink-salmon or white bloom of fungal sporulation may appear. Fusarium oxysporum occurs in specialised forms (formae speciales), each of which infects only a particular species or genus of plant (e.g. tomato, cucumber, pea, carnation, banana), which helps to distinguish it from pathogens with a wide host range.

Treatment

A vascular disease is practically impossible to cure, so infected plants should be removed and destroyed together with the root ball. Under cover, the substrate is disinfected or replaced, and fungicides have only limited effect against the pathogen hidden within the vascular bundles. Removed plants are discarded with the waste or burned — not composted, because the fungus survives in the debris. After handling infected plants, tools, pots, staking and tunnel structures are disinfected so as not to carry spores to healthy sites. Under cover, thermal disinfection of the substrate (steaming) or its complete replacement is effective, and outdoors — soil solarisation during the hottest part of the year. Biological antagonists (fungi of the genus Trichoderma, non-pathogenic strains of Fusarium, Bacillus bacteria) introduced into the root zone provide support — but they act mainly preventively, colonising the roots before the pathogen does, and will not rescue already infected plants. In crops of solanaceous and cucurbit vegetables (tomato, cucumber, melon, watermelon) a practical solution is grafting onto a resistant rootstock. It is also worth reducing ammonium-nitrogen fertilisation in favour of the nitrate form and keeping the soil pH from neutral to slightly alkaline — both measures ease the course of the disease. Systemic fungicides used for seed dressing and watering transplants provide protection only at the emergence stage; against the mycelium enclosed within the vascular bundles of an adult plant they are powerless.

Prevention

Growing resistant cultivars, crop rotation (the fungus survives for a long time in the soil), using healthy seedlings and disinfected substrate, avoiding root injuries and nitrogen over-fertilisation and keeping soil pH and temperature unfavourable to the pathogen. The foundation is choosing resistant or tolerant cultivars — for tomato these are cultivars marked with the fusarium wilt resistance symbol. Crop rotation must be long, because the fungus survives in the soil for many years as resting chlamydospores; a break of several years should be kept between crops of the same susceptible species, sowing non-host plants in the meantime. Seedlings are obtained from healthy, certified material and produced in disinfected substrate and clean trays. The roots must be protected from injury — every wound is a gateway for the pathogen; the presence of root nematodes (e.g. the root-knot nematode) is especially dangerous, as by puncturing the roots they open the way to infection and markedly increase the severity of the disease, which is why controlling them is part of fusarium wilt prevention. Good site drainage, avoiding overwatering and overheating of the soil, moderate nitrogen fertilisation (with a predominance of the nitrate form) and keeping the soil pH from neutral to slightly alkaline (fusarium wilt intensifies on acid soils) all promote plant health. Soil solarisation and biodisinfection with brassica plant residues (biofumigation) before establishing a susceptible crop can also help.

How to tell it apart from look-alikes

Control — organic and chemical methods