The influence of temperature and snow cover on colonization of red pine by Gremmeniella abietina

Potential physical and physiological factors influencing mid-winter colonization of host tissue by Gremmeniella abietina, the pathogen inciting Scleroderris shoot blight, were investigated. Ambient air temperature, or temperature as modified by a snow cover, was identified as an important limiting factor in the development of disease on red pine in the Lake States. The concept of a conducive day was developed to quantify the effect of temperature and snow cover on disease development. A conducive day was defined as one in which the temperature remains between {dollar}-6{dollar} and +5{dollar}spcirc{dollar}C, or the host tissue is completely covered with snow. Forty-four or more conducive days between the time of inoculation and 28 February allowed disease development on inoculated trees in this study. This was true of inoculated trees exposed to natural field conditions, artificially manipulated field conditions, and completely artificial conditions.; This model provides an explanation for the occurrence of the disease syndrome caused by the NA serotype of Gremmeniella abietina (1) only on small trees or lower branches of large trees, (2) only in areas north of about 45{dollar}spcirc{dollar} N latitude, and (3) only after a latent period that includes one or two winters following initial host penetration by the pathogen. The conducive day model, incorporating a 2-yr latent period and temperatures not modified by snow, also provides an explanation for the disease epidemic in upper crowns of large trees in New York State in the mid-1970s, caused by the EU serotype of the pathogen, and the absence of similar symptoms in upper crowns of large trees in the Lake States. A case was presented for a similar response of the two serotypes to environmental conditions affecting disease development.; A reliable technique for producing typical disease symptoms of Scleroderris shoot blight under completely artificial conditions was developed. Seedlings held at 4{dollar}spcirc{dollar}C subsequent to initial fungal penetration developed disease symptoms in as few as 53 days after inoculation.; The conducive day model was shown to have potential applicability to disease situations throughout North America and Europe. Regions of northwestern United States and western Canada have susceptible host species and climates which would be conducive to disease development, should the pathogen spread or build up in these areas. Therefore, efforts to prevent future westward spread of the pathogen are advised.