Didymella ligulicola (ray (flower) blight of chrysanthemum)

All plant parts, including roots, may be attacked, but flowers and cuttings are particularly susceptible (Fox, 1998).On cuttingsCuttings are usually attacked at the terminal bud, whence infection spreads downwards to the whole plant. Unopened buds, bracts and stem tissue become darkened. On leaves, the fungus causes irregular brownish-black blotches, 2-3 cm across. Under favourable conditions, these rapidly coalesce and the leaf rots. On stems, symptoms are associated with positions where the diseased leaves adjoin, with wounds, or at the cutting base. During rooting, symptom development may be arrested, but diseased tissues remain on the plant and constitute a dangerous source of inoculum.On adult plantsStem lesions, which may girdle the stem and are often localized at the base or nodes, are associated with an abnormal appearance in the corresponding shoots, without the latter being contaminated by the fungus. This is due to production of a phytotoxin which induces drooping of terminal growth, makes leaves smaller, chlorotic and more or less brittle, and causes slight dwarfing.On flowersFollowing infection, spots develop, initially on one side of the blossom only. The spots appear reddish on light-coloured cultivars and brownish on darker ones. Infection subsequently spreads rapidly and complete rotting of the flower head may occur, the infected florets sticking together. The fungus then grows down the peduncle, blackening and weakening the tissue, so that the head eventually droops and wilts.

In Europe, the disease has been controlled successfully with benomyl. However, the repeated and excessive usage of this fungicide over a number of years has led to resistance build-up and a consequent increase in ray blight importance (Grouet, 1974). More recently dicarboximide derivates have been successfully used for control (Engelhard, 1984).Currently there is no biological control method available. However, certain cultivation and phytosanitary requirements can reduce infection by D. ligulicola, especially during the rooting of cuttings (Hahn and Schmatz, 1980).

The disease was recorded in North Carolina, USA, in 1904, and remained localized and of little importance until the late 1940s when, concurrent with the intensification of chrysanthemum flower and pot plant production, it began to cause serious losses throughout the range. It is now considered the most serious fungal disease of chrysanthemums in Florida. In 1975, in Connecticut, the disease was reported to be particularly important on chrysanthemum cuttings in propagating benches under mist; 50% losses occurred.The increasing intensification of chrysanthemum production, with all-the-year-round cultivars, mist benches, use of dark covers, etc., favours spread and development of the disease. In addition, the fungus can develop under a wide range of conditions and, once established, is both difficult and costly to eradicate. The fact that the disease is recorded in California shows that it will persist even in areas with apparently unfavourable climatic conditions.