Mycosphaerella laricis-leptolepidis (needle cast of Japanese larch)

In early July, scattered brown spots (5-7 or up to 20 per needle), surrounded by a faint chlorotic halo, appear on needles of the crown. The needles of the upper branches are often less infected than those of the lower ones. Lesions gradually coalesce, attaining a width of 1 mm or more and cause the needles to go brown and the tree to have a scorched appearance. This coloration is particularly marked in summer and autumn. Before the needles are cast, black pustules, spermogonia, appear on the upper surface of the dead area. Needle cast results in trees with all or portions of their crowns thin and the remaining needles are confined to tufts at the end of the branches. Needles from susceptible trees have less chlorophyll, less N, P, K, and more Ca and Si than resistant ones. Nitrogen content falls in the autumn in resistant needles but increases in susceptible, infected needles. Repeated defoliation results in a decrease in growth increment and death of shoots and twigs. In general, trees in plantations are most severely affected but seedlings and saplings may also be attacked. Trees in mixed hardwood stands are usually less affected.For additional information, see Ito et al. (1957), Peace (1962), GFES (1965), Pyun and La (1970).

In Japan, chemicals are not usually applied in plantations but mancozeb in six 2-weekly applications has given some control. Three to four sprays with copper fungicides during June-July has proved effective in preventing disease development. In addition, control can be aimed at the infection source by removing or burning diseased fallen needles in spring. Resistant clones are also used (Kobayashi, 1980). A forecasting model has been developed in Manchuria (Chang et al., 1990).

Since the early 1950s, this fungus has increased in prevalence and, although disease severity varies widely between forests, it is the most important defoliator of Larix leptolepis in Japan. Usually, 10 to 20-year-old forests are severely infected. Up to 80% reduction in wood volume occurs in heavily infected trees. In Japan, M. laricis-leptolepidis can also attack introduced L. decidua (Imazeki and Ito, 1963). Wang et al. (1999) describe M. laricis-leptolepidis as a major pest of Larix olgensis in Manchuria.