Early blight of potato (Solanum tuberosum) is caused by Alternaria solani and occurs annually to some degree in Idaho. The timing of its appearance and rate of disease progress determine the impact on the potato crop. Though losses rarely exceed 20%, they can be higher and occur in stored potatoes if the disease is not controlled. Early blight is managed mainly by cultural practices such as plant nutrition, water management, and avoidance of plant stress, but also with the use of foliar fungicides. Currently, the main fungicides labeled for control of early blight are in the carboxamide and strobilurin fungicide groups. Development of resistance to some fungicide groups may contribute to the loss of control of early blight. Isolates of A. solani from Idaho potatoes were tested for resistance to boscalid in the carboxamide group. Diseased potato leaves with early blight symptoms were collected from fields near Parma, Rupert, and Aberdeen in southern Idaho in 2009 and 2010 and Bonners Ferry in northern Idaho in 2010. To obtain A. solani isolates from leaves, small pieces of leaf tissue (5 × 5 mm) were taken from the center of early blight lesions and streaked across the surface of a thin layer (3 mm) of water agar. Plates were incubated at 25°C overnight to allow spores to germinate. Single germinated A. solani spores were transferred to acidified potato dextrose agar and incubated in the dark at 25°C. Germinated spores were identified based on spore morphology. Spores of A. solani can be easily distinguished from other Alternaria spp. found on potato because they are ellipsoid to oblong and taper to a long beak that is usually as long as the spore body. The identity of cultures grown from single spores was confirmed by colony and spore morphology. Sensitivity of A. solani isolates to boscalid was determined by the spiral gradient endpoint method (2). For all isolates, the effective concentration for 50% reduction in growth was outside the range of the spiral plate dilution series (i.e., isolates were either completely insensitive or completely sensitive to boscalid). In total, 46 isolates (20 collected in 2009 and 26 collected in 2010) were tested against boscalid. Experiments were carried out twice with 2009 isolates using mycelial strips and conidial suspensions. Experiments with 2010 isolates were carried out three times using only conidial suspensions. Of the isolates from 2009, 15% were insensitive. There was no difference between the use of mycelial strips or conidial suspensions. In 2010, 62% of isolates were insensitive. By location, 72% of isolates from Parma, 73% from Rupert, 63% from Aberdeen, and 44% from Bonners Ferry were insensitive. Resistance to boscalid has been reported in A. alternata isolates from pistachio (1,3). However, to our knowledge, this is the first report of resistance to boscalid in isolates of A. solani on potato. These data suggest that resistance to boscalid is widespread in Idaho, even in areas like Bonners Ferry where potato cultivation is limited. Boscalid insensitivity in vitro may not translate directly to commercial production and currently there is no evidence to suggest that boscalid has failed to control early blight in Idaho. However, the discovery of insensitive isolates suggests that boscalid should be considered at high risk of resistance development.
References: (1) H. Avenot et al. Plant Dis. 91:1345, 2007. (2) H. Förster et al. Phytopathology 94:163, 2004. (3) N. Rosenzweig et al. (Abstr.) Phytopathology 93(suppl.):S75, 2003.