Verticillium fungi: Considering grassing over area for 15 years
Question: Can you please advise me as to why my Cotinus smoke bush has gone like this? It faces south in a windy spot. It is five years old and I have bark mulch at the bottom. Would it be a good idea to prune it low for a better chance this year? (Mrs BA Collins, North Walsham)
Looking at the stems and the way that it has died back this could well be Verticillium wilt, caused by the soil-borne fungi Verticillium dahliae and V.alboatrum. Both infect a very wide range of garden plants through the roots and then grow upwards in the water-conducting tissues, causing wilting of the upper parts due to water stress. Wilting is mostly seen from spring until autumn.
Plants most often affected include Chrysanthemum, carnation, aubergine, potato, tomato, cucurbits and strawberries. Woody plants are also affected, including Acer, Cotinus, Rhus, Berberis, Catalpa, Cercis and Rosa, but the full host range is very wide indeed. Conifers are not affected.
Plants that are affected may show the following symptoms: Yellowing and shrivelling of lower leaves; some or all of the plant suddenly wilting, especially in hot weather.
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Plants may recover in cooler or wetter conditions; brown or black streaks in the tissue under the bark.
These are visible as a circle of brown marks, if the stem is cut across transversely. In woody plants the marks are in the outer growth ring. Branch dieback is a common symptom. Sometimes only part of the plant may wilt.
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In woody plants verticillium wilt shows as brown or black streaks in the tissue under the bark.
Verticillium fungi reside in the soil for many years. These germinate and penetrate into the roots of susceptible hosts, where they grow into the water conducting system (xylem). The fungus can be spread in contaminated soil, so if the disease is suspected, be careful not to spread soil on tools or muddy boots.
Weed control is important, because some weeds are hosts and some may not show any visible signs of infection. Where the disease is confirmed, remove the infected plant with as much root system as possible and destroy, taking care not to spread soil. Consider grassing over the area for at least 15 years, or plant a resistant replacement.
Heavy watering and application of ammonium-based fertilisers (not nitrates) may stimulate the production of new conducting tissue and help them recover, but this does not guarantee that reinfection will not occur in future years.
Avoid replanting with susceptible plants, especially Acer. Resistant plants include: Betula, Cercidiphyllum, Crataegus, Fagus, Gleditsia, Liquidambar, Morus, Platanus, Salix. Conifers are effectively immune.
•This article was first published on January 14, 2012.