Needle blight of eastern white pine (Pinus strobusL.) is characterized by an orange-red discoloration of the distal portions of current year needles. At Chalk River, Ontario, 600 seedlings and saplings of white pine were examined repeatedly throughout the 1957 and 1958 growing seasons for the appearance and development of needle blight symptoms. The first of these were faint pinkish spots on the stomata-bearing faces of needles in semimature (about four-week-old) tissue. These developed rapidly into orange-red bands which within a few days spread to the needle tips. Mature tissue was not susceptible to attack, so lesions which developed subsequently were always proximal, on younger tissues derived from basal meristems. Attacks which occurred in the early summer killed only limited areas at the tips of needles, whereas those that occurred when the needles were nearly full-grown involved most of their length. Needle blight incidence was confined to a few major outbreaks during one season and in each of these many trees developed typical symptoms at essentially the same time. Each of these major outbreaks of the disease occurred after 1 or more days of wet weather which was followed suddenly by a continuous sunny period. No microorganisms were isolated from tissues displaying the initial needle blight symptoms.The data with respect to the nature and occurrence of needle blight are believed to suggest that susceptibility to the unfavorable conditions which incite it is inherent in the individual and that differences in response among members of a local population depend on variations in susceptibility, rather than on a varying local predisposition among uniformly susceptible individuals. The blight is initiated in semimature leaf tissues only but then spreads distally throughout adjacent, more mature tissues with a similar pattern of breakdown.
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