On a Needle Blight of Pine Caused by <i>Rhizosphaera kalkhoffii</i> BUBÁK Life History, Physiological Characteristics and Pathogenicity

In early summer of 1965, a severe damage of needle blight caused by Rhizosphaera kalkhoffii BUB, was observed on Japanese red pine, Pines densiflora SIEB. et ZUCC., in the central and eastern parts of Japan.<br> The present paper deals with symptoms and signs of the disease, and life history, physiological characteristics and pathogenicity of the causal fungus.<br> The first evidence of infection is the presence of very small yellowish to grayish-green spots around the stomata of the newly developing needles in late May. Discolorations rapidly enlarge in the apical portions of the needles and may involve from one-third to two-third of the entire length of needle with a sharpline demarcation between injured and healthy tissues.<br> About 10 days after the evidence of lesions, pycnidia of the fungus appear on the stomatal lines of the upper and lower surfaces of the needles. Pycnidia and pycnospores are abundant during May to October except August, and are found even during winter (Fig. 2).<br> Searches for perfect stage of the fungus have not succeeded, but it seems that the fungus commonly overwinters as pycnidia, pycnospores and dormant mycelia within tissues of affected needles.<br> Pycnospores germinate in two different ways: (1) In a budding way, secondary conidia increase in numbers and then hyphae grow from the mass of these conidia, and (2) Normal type of germination, i.e., germ tubes develop directly from one or both ends of pycnospore. As shown in Table 1, the percentages of the normal type of germination show the ascending pattern with increase in the H-ion concentrations. Pycnospores start to germinate within 6 hours at 25°C on agar medium. The germination occurs between 5_??_30°C, with an optimum at 25°C (Table 2).<br> The mycelial growth of the fungus takes place between 0_??_30°C, with an optimum at 25°C (Table 3). The mycelia of the fungus grow well in potato decoction, pine needle decoction and WAKSMAN's solution, but poor in RICHARDS' solution. Addition of thiamine up to 100μg/ι to RICHARDS' solution increases the mycelial growth of the fungus, though no remarkable effect is observed at higher concentrations of thiamine (Table 5).<br> Abundant conidia are produced in the colony grown on the various kinds of agar and solution media. Production of conidia also takes place through the direct budding way from the mycelia or the conidia themselves.<br> From the results of a series of inoculation experiments (Table 7), it is reasonable to conclude that the pathogenicity of Rhizosphaera kalkhoffii is considerably weak to the current season's needles of Pinus densiflora growing under the normal conditions.