A PRELIMINARY STUDY ON CYTOSPORA CANKER DISEASE OF PINUS ARMANDI

Cytospora canker diseas of Pinus armandi was first found out during the winter at the end of 1983 to the beginning of 1984.It occurs commonly on the plantations of P.armandi within the natural range of this pine tree in the Northeastern Yunnan from which there has been no recording before.This disease is restricted to high elevations,being most prevalent in P.armandi forest from the altitude of 2500 to 3000 meters.The canker develops either on the branches or on the main stems of P.armandi.The incidence of infected trees may be up to l00 per cent and its utmost disease index may attain to 56,3.An infected stand of P.armandi composed of 15 years old trees had been a 17.4 per cent mortality.Consequently,this is one of the most prevalent and destructive diseases of P.armandi.The elongated cankers formed on the affected trunk or branches arc slightly swollen areas and can be identified easily by their coating of resin.By cutting into the bark it will reveal brown areas of killed bark and cambium.Small black pimples appaer on the dead bark on the canker surface and from which exude long thin threads of spores during wet weather.Dying branches were first exhibiting yellow-green foliage then turning into purple at last the pine needles would be cast off.The canker disease of P.armandi is caused by Cytospora pini Desm.The overwintering of this fungus is by way of forming fruiting bodies on bark and as vegetative mycelium in cankers.During wet weather,in the spring fruiting bodies exude thin threads of spores which will be washing to other branches by rainsplash.Field observations indicate that the fungus is weakly parasitic when the host has been in unfavorable condition under certain causes,such as drought,winter injury,insect activities,sheep grazing and mechanical injuries or so which will weaken the growing situation subjected to this fungus.The disease is worst in overcrowded stands,especially on the young ones which would be more damaged.Susceptibility of these pines to canker disease are decreasing with increasing ages.This disease can be effectively prevented by keeping trees as vigorous as possible.Cares should be taken in the selection of sites for plantations and also,an appropriate thinning cycle and thinning intensity should be carefully considered.All cankered young trees and dead branches of larger trees should be removed in the course of thinning.Carry out a rational grazing plan in the stands.All of the unnecessary wounding of the trees should be avoided.Those plantation areas which have been badly affected by this disease should be isolated by sparying with fungicides round them t