Verticillium wilt

Symptoms

The first symptom is a sudden wilting and drying of leaves on individual branches or on one side of the crown, while the rest of the plant appears healthy. A cross-section of an infected shoot reveals a brownish or greenish discolouration of the wood — the vascular bundles clogged by the fungus, which can no longer conduct water. The disease progresses over successive years, affecting an ever-larger part of the crown and leading to the gradual death of the plant. Maples (especially Japanese maple), catalpa and many other trees, shrubs and perennials are particularly susceptible. The disease takes two forms: an 'acute' form, in which a branch or the whole plant dies rapidly within a few weeks, and a 'chronic' form — a slow decline spread over several seasons, with smaller, paler leaves, marginal scorching of the blades (resembling sunburn) and premature autumn colouring on infected shoots. The sectoral, one-sided distribution of symptoms is the hallmark of Verticillium wilt above all on woody plants — the part of the crown supplied by the clogged vessels dies off. The discolouration of the wood is best seen just under the bark as longitudinal streaks, and in cross-section as an interrupted ring; in maples it is often olive-green. Symptoms intensify in heat and during drought, when the demand for water exceeds the capacity of the damaged bundles. On herbaceous plants (dahlia, aster, chrysanthemum, tomato, potato, strawberry) one observes wilting and yellowing of the lower leaves, stunting and one-sided (half-blade) yellowing of the leaf — on these species, however, the symptom picture alone is unreliable for differentiation, because one-sided yellowing may also be caused by Fusarium wilt. The vascular discolouration by itself is not conclusive — a definitive diagnosis requires laboratory isolation of the pathogen.

Treatment

There are no effective chemical agents against this soil-borne fungus, which enters the plant through the roots. Infected branches should be cut back to healthy tissue, and the plants strengthened by moderate watering in drought and balanced feeding, which is often enough for the tree to contain the infection on its own. Severely and persistently infected plants must be removed in extreme cases. Make cuts clearly below the discoloured wood, and disinfect tools between successive cuts and between plants, because the fungus spreads with the sap and within the wood. Carry cut shoots and removed plants, together with the root ball, out of the garden and dispose of them with the waste (or burn them, where local regulations allow) — do not compost them, because resting microsclerotia form in the dead tissue. Avoid heavy nitrogen feeding, which stimulates lush, susceptible growth; opt instead for balanced feeding that includes potassium, and for mulching that retains moisture. Trees and shrubs are able to 'wall off' (compartmentalise) the infected vessels and, after a few seasons, put out healthy growth, so with a milder infection it is worth giving the plant time rather than removing it straight away. Herbaceous plants and vegetables are removed whole. Chemical disinfection or soil fumigation matters only in professional nursery production and is not a realistic option in the garden.

Prevention

In sites where plants affected by Verticillium wilt previously grew, susceptible species should not be planted, because the fungus survives in the soil for a long time. It is advisable to use only healthy, inspected nursery stock and to avoid damaging roots during garden work. Microsclerotia remain viable in the soil for well over a decade, so after susceptible vegetables use a long rotation with resistant species (cereals, grasses, maize), and plant the empty site left by a diseased tree with resistant plants — conifers, ginkgo, birch, beech, oak, hornbeam, hawthorn, apple or pear — instead of another maple, catalpa, sumac, dahlia or tomato. Remove weeds, because many of them are symptomless carriers that build up a reserve of inoculum in the soil. Take care of drainage and soil structure, do not compact the soil, and do not move soil or tools from infected sites to healthy ones. Limit nitrogen feeding and do not allow the plant to dry out or become waterlogged, because stress predisposes it to infection.

How to tell it apart from look-alikes

Control — organic and chemical methods

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.