In August 2002, white leaf spot disease was observed on red kidney bean (Phaseolus vulgaris L.) at three fields near Staples, MN. Disease incidence in these fields, calculated as the proportion of plants showing visible symptoms, ranged from 20 to 100%. Symptoms and signs consisted of abundant, white sporulation on the abaxial side of leaves that was limited by secondary veins, resulting in angular-shaped lesions. Corresponding with fungal growth, yellow, angular spots were observed on the upper leaf surface. In older leaves, lesions coalesced, covering most of their surface, while younger leaves were relatively free of symptoms. Heavily diseased plants senesced faster and defoliated earlier than nonsymptomatic plants. Identification of the pathogen was conducted by direct observation of infected plant tissues with light and electron microscopes. Conidia were hyaline, filiform, rounded at their apex, and with no visible scar at their point of attachment to the conidiophore. Conidia had one to four septa, were 57 to 68 μm long, and approximately 2.4 μm in diameter. Conidia were produced at the tip of colorless, short conidiophores that emerged through stomata in groups of five or more. These traits are in agreement with the description of Pseudocercosporella albida (Matta & Belliard) Deighton (1,2). A sample of infected tissues has been deposited at the U.S. National Fungus Collection (BPI 842303) at Beltsville, MD. The impact of this disease on bean yields in Minnesota was not estimated. However, in the cool highlands of Colombia, where white leaf spot is endemic, yields can be reduced by as much as 47% (3). To our knowledge, this is the first report of the presence of this pathogen on dry bean in North America.
References: (1) F. C. Deighton. Trans. Br. Mycol. Soc. 66:547, 1976. (2) A. Matta and L. Belliard. Phytopathol. Mediterr. 12:102, 1973. (3) H. F. Schwartz et al. Plant Dis. 65:494, 1981.
