Control of potato early blight with triazole fungicide using preventive and curative spraying, or a forecasting system
During the 2005 and 2006 growing seasons, two field trials were conducted at the INTA Balcarce Experimental Station (Argentina) to study the control of potato early blight (Alternaria solani). Uncut virus-free seed tubers of potato (Solanum tuberosum) cv. Pampeana PNTA that were susceptible to early blight and resistant to late blight were machine planted in 0.20-m intervals in rows spaced 0.8 mapart. Each plot consisted of 4 rows that were 5 meters long. Difenoconazole (Bogard 25EC) was sprayed weekly at a rate of 0.25 L· ha-1 or bi-weekly at a rate of 0.50 L· ha-1 on a preventive program and on a curative program. The same treatments and rates were also applied when the forecast model (Specware 6.02) reached 300 P-Days after emergence, and applications were then repeated thereafter every 7 EAST severity values. Disease severity was rated using the 0-50 rating scale (0 = no infection; 50 = maximum infection). Tubers from the two center rows of each plot were harvested and graded into two categories, either marketable (>60 g) or undersize tubers "seeds" (<60 g), and weighed for total yield. Climatic conditions were very favorable for disease development. In both years, untreated plants showed severe early blight symptoms and reached the maximum disease severity. Preventive fungicide applications at the high dose and applications following the Specware 6.02 disease prediction model showed similar efficacy. Curative fungicide applications at the lowest dose and applications on the basis of the Specware 6.02 disease prediction model resulted in a poor control of foliar symptoms, and yields were lower than those obtained with a preventive control program.