Suppression of potato early blight (in situ) and germination of Alternaria spp. Conidia (in vitro) with strobilurin fungicides

Early blight, caused by Alternaria solani Sorauer, is a ubiquitous disease that may limit potato yield. Adequate crop fertility and appropriate fungicide applications usually retard development of this disease. Strobilurin analogs, such as azoxystrobin, represent a novel highly selective group of fungicides that inhibit mitochondrial respiration in many phytopathogenic fungi. The objectives of this research were: (1) to ascertain the efficacy of azoxystrobin and pyraclostrobin, in terms of potato early blight control and enhanced yield and quality, by comparing them to a conventional fungicide program, (2) to determine if potatoes treated with these two strobilurin chemicals require reduced nitrogen (N) fertilizer inputs, and (3) to verify the in vitro sensitivity of A. solani isolates from PEI to azoxystrobin.; Results from this study indicated that both strobilurin products were linked to significantly higher biological yields for Russet Burbank in 2003 and Shepody in 2004 when compared to plots receiving no strobilurin fungicides. In 2003, no early blight symptoms developed despite artificial inoculation. Thus, in the absence of early blight infection, these increases in yield may be attributed to the physiological and developmental alterations that strobilurin products have on treated plants. No significant differences in biological yield based on N fertility were observed.; In 2004, defoliation induced by artificial inoculation was visually apparent in the controls, while symptoms did not develop in strobilurin-treated plots. For this reason, the significant yield increases observed in the Shepody plots may be attributed not only to the beneficial effects of strobilurins, but also to the control of early blight. In addition, the severity of early blight was low at both rates of N fertility in azoxystrobin and pyraclostrobin plots in 2004. Growers, therefore, may subsequently lower N inputs without compromising early blight control.; Upon examination of the 2004 Russet Burbank check plots, there was an obvious N fertility effect with the higher rate having a reduction in foliar disease severity. Furthermore, the artificial inoculation study revealed that Russet Burbank control plots had a lower disease severity rating compared to the Shepody controls.; Isolates of A. solani and A. alternata, causal agent of potato brown spot, were collected in 2003 and 2004 from several potato growing areas throughout PEI. A spore germination assay was used to determine the baseline in vitro sensitivity of these two pathogen populations to azoxystrobin. EC50 values ranged from 0.002-0.022 mug/ml for the 17 A. solani isolates tested, while the values for the 23 A. alternata isolates tested ranged from 0.001-0.028 mug/ml. These values were much lower than the EC50 value of 0.258 mug/ml that was determined for a known insensitive isolate from North Dakota, US - an area where widespread use of azoxystrobin has caused a shift in sensitivity.