WATER SOAKING OF LEAVES IN RELATION TO DEVELOP- MENT OF THE BLACKFIRE DISEASE OF TOBACCO ^ By E. E. CLAYTON Senior pathologist^ Division of Tobacco and Plant Nutrition, Bureau of Plant Industry, United States Department of Agriculture INTRODUCTION

In a previous publication ^ the general situation with respect to the wildfire and the blackfire or angular leaf spot diseases of tobacco (Nicotiana tabacum L.) was discussed and the need for explaining the occurrence of leaf spot epidemics was indicated. It was shown that epidemics of wildfire {Bdcterium tabacum Wolf and Foster) are conditional on the occurrence of leaf water soaking, and the factors modifying leaf resistance to water soaking were discussed. These results are extended in the present paper by simüar studies with the blackfire (Bad. anguLatum Fromme and Murray) disease. Blackfire is the common tobacco leaf spot disease of Virginia, North Carolina, Tennessee, Kentucky, and Wisconsin. Following its identification by Fromme and Murray,^ it was long regarded as established that the disease was caused by Bacterium angulatum and was serious and destructive. It was a fact, however, that no one had ever produced the "epidemic'' type of blackfire (see fig. 2, A) under controlled conditions. Inoculations produced only small localized lesions (fig. 1), and in consequence Valleau ^ suggested that the epidemic type of leaf spot was nonparasitic in nature. It is beheved that the results presented in this paper adequately clear up this situation.