THE BEECH BARK DISEASE: A NECTRIA DISEASE OF FAGUS, FOLLOWING CRYPTOCOCCUS FAGI (BAER.)

Abstract

This paper is concerned with a disease of beech (Fagus) caused by the sequent activity of a scale insect and a parasitic ascomycetous fungus. Although the writer's studies have dealt almost exclusively with the American beech (F. grandifolia Ehrh.), the insect has been observed on F. grandifolia var. caroliniana Fern, and Rehd., as well as on the European beech (F. sylvatica L.) and a number of its varieties. The name "beech bark disease" is proposed.The European history of this disease, gleaned largely from the literature on the insect, is reviewed, beginning with the first reports of injury at the middle of the last century in England and Germany, tracing its reported activity to the present, and the conclusion is reached that the disease is not now regarded as so serious an affliction of beech as formerly and that it is widespread and enphytotic in central and western Europe. In America the disease has been active for approximately 15 years; it first attracted attention in the vicinity of Halifax, Nova Scotia, and has spread steadily until it is now active throughout Nova Scotia, in southern New Brunswick, and in localized areas of eastern and south central Maine, although the insect pathogen has been present also for perhaps a decade in eastern Massachusetts. A pathological line-plot cruise of typical affected areas in Nova Scotia and New Brunswick demonstrated that, in the stands examined, the beech infected by the fungus had attained a uniform mean of 90% and that in the regions infested for a longer period approximately 50% of the beech had been killed at the time of observation. The conclusion is reached that the disease in America is assuming the proportions of a fatal and spreading epiphytotic.The first indications of the disease are the appearance of the minute insect on the bark of trunk and branches, and the subsequent development of fruiting bodies of the fungus in the areas previously occupied by the insect. The insect probes the living tissues of the outer bark, extracting protoplasmic materials and causing the death of punctured cells. The shrinkage of groups of killed cells leads to tearing of the periderm, which enables the fungus to initiate infection. Once past the barrier of phellem, the fungus is able to advance through the living tissues of bark, cortex, phloem, cambium, and sapwood. Death of the infected tissues interferes with normal conduction and storage in the trunk and results in a progressive killing of the tree, which becomes complete when the fungal lesions have coalesced sufficiently to effect an interruption in conduction. As infestation progresses, the foliage and twigs dry and die, whole branches cease to leaf out, and large areas of bark on the trunk crack, usually loosen from the wood, and eventually fall away. On younger trees infection is less abundant, the fungus advances less readily, and individual lesions frequently become surrounded by healthy bark and wood, and eventually form depressed areas but never typical scalariform cankers.The insect (Cryptococcus fagi [Baerensprung]) which initiates this disease has been reported from Germany, Switzerland, Czechoslovakia, France, Belgium, Holland, Sweden, the British Isles, eastern Canada (where it was first reported in 1914 and has probably been present since 1890), and the northeastern United States (where it was discovered by the present writer in 1929). Synonymy, previous descriptions of the insect (a member of the Homopterous family Coccidae), and its life history are reviewed, with personal notes on habits, dissemination and long range distribution, and factors influencing infestation.The fungus following the insect in America is a species of Nectria in the coccinea group as defined by Wollenweber. Its specific identification must await a thorough comparative study of the American forms, but spore measurements indicate that it is an undescribed variety of Nectria coccinea (Pers.) Fries. A brief description is given. The fungus has been repeatedly isolated and grown in pure culture on a variety of media, producing conidia and ascospores. The influence of low temperature, aeration, and hydrogen ion concentration are discussed and data given on the height of ascospore discharge. Spore-trap experiments demonstrated that ascospore discharge follows rains of sufficient duration to wet the bark and continues until after the surface of bark and perithecia appears dry; both ascospores and macroconidia are air borne.The role of each of the pathogens in producing the disease has been demonstrated. The evidence consists of the following: (1) the constant association of the organisms with the disease, (2) absence of the disease when one only is present, (3) consistent isolation of the fungus from infected tissues, and (4) the inability of the fungus to infect tissues not infested by the insect and its ability to infect those infested. The insect probes the bark, resulting in a rupture of the periderm attendant on death and shrinkage of the living cells of the bark on which the insect feeds. This enables initial infection by the fungus. The fungus grows parasitically, causing the death and destruction of the storage and vascular systems of the trunk and branches.The fungus is shown by experiments to be able in general to enter and infect extensively only bark on which the insect has been present for approximately a year or longer. Observations in each of three consecutive years on tagged trees in permanent sample plots show that, once the trees become generally infested by Cryptococcus fagi, Nectria infection develops on the majority within three years and kills some of them in one or two years. Later, almost the entire stand becomes infected and the amount of infection builds up rapidly, with resulting mortality.A pathological line-plot cruise of 200 plots containing 4,483 beech shows on analysis small but definite positive correlation between percentage of beech infected by Nectria, and percentage of beech in stand, position on ridge, and steepness of slope; severity of infection shows definite correlation with diameter (d.b.h.),* and questionable correlation with forest type, and crown density. Mortality, expressed as percentage of beech (over 3 in. d.b.h.) killed, is correlated with diameter, crown class, and position on ridge, and questionably with forest type, and steepness of slope.The disease can be controlled on ornamental trees by early eradication of the insect with insecticides. Control in forest stands should aim first at the salvage of infected, dying, and recently killed timber, for the purposes of obtaining the greatest possible return on a rapidly depreciating investment and of preventing the development in the slash of unsanitary conditions conducive to further deterioration in the remaining stand. Second, control should aim at the possibility of combatting the insect pathogen with fungal and insect enemies. Third, it should, by forest management, favor beech on broad ridge tops rather than steep slopes, and cull the larger trees in an attempt to produce changes in the environment designed to restrict activity of the pathogens and substitute a younger, less susceptible stand.