Plants affected
A wide range of plants is affected including ornamentals, trees, vegetables, fruit, and field crops. The most susceptible tree species (based on numbers of samples received in the Pathology Laboratory at Wisley) are Acer (maples), Cotinus (smoke bush), Catalpa (Indian bean tree), and Cercis (Judas tree).Symptoms
Inconsistent wilting occurs in particular stems or branches
of a plant. The whole plant can also be affected. Wilting
can occur very suddenly, especially in hot weather. This can
lead to dieback and discoloration of the inner tissues of
the stem.
Green or olive-brown spots or blotches are found under the bark of affected woody plants, in cross-sections of the stem, following the annual rings of the plant. These may appear as streaks in the wood if the bark is stripped away.
Cause
Wilting is due to toxic substances produced by the fungus Verticillium, which infects roots and then spreads upwards in the conducting tissues. Gums produced by the plant as a defensive reaction block the plant tissues and prevent the passage of water through the plant.
Control
There is no chemical control for this disease. Infected plants or parts of plants should be removed, and disposed of, but not on the compost heap.
The fungus can be spread in soil on spades and muddy boots, and possibly also on pruning tools, so clean all implements thoroughly with disinfectant. The fungus can remain in infected soil for years, so once the disease has been discovered, resistant plants should be planted.
Lightly affected plants may recover. This can be aided by application of nitrogenous fertilisers (but not nitrates) to boost the growth of new wood, which may remain disease free.
Resistant plants
Alnus (alder), Betula (birch), Carpinus (hornbeam), Cercidiphyllum, conifers (which are completely immune to infection), Crataegus (hawthorn), Eucalyptus, Fagus (beech), Ginkgo, Gleditsia, Ilex (holly), Liquidambar (sweet gum), Morus (mulberry), Platanus (plane), Populus (poplar), Pyrus (pear), Quercus (oak), Salix (willow), Sorbus (not S. torminalis), Zelkova.