Abstract
ABSTRACT Ramularia leaf spot (RLS) is the major cotton disease in Brazil and one of the main causes of crop yield reduction. The disease causes defoliation reduces photosynthetic capacity and causes premature opening of bolls. Due to the impact of RLS on the production of Brazilian cotton and the lack of scientific publications investigating the disease, the present work aims to evaluate the efficiency of fungicides in controlling disease and their impact on yield and fiber quality. Three field experiments were carried out during the 2014/2015 and 2016/2017 seasons in Planaltina, Federal District and Cristalina, Goiás. Experiments followed a randomized complete block design (4 blocks) and treatments consisted of the application of fungicides, plus a non-treated plot. The fungicide applications began preventively and were repeated every 14 days until six sprays had been completed. Severity of RLS was assessed and used to calculate the area under disease progress curve (AUDPC). Yield and cotton fiber quality were also evaluated. All fungicides had reduced AUDPC and obtained higher cotton yields in relation to the non-treated plot. Strobilurin-group fungicides (azoxystrobin, pyraclostrobin and picoxystrobin) showed low effectiveness when sprayed alone. The demethylation inhibitor fungicides (difenoconazole, epoxiconazol, tetraconazol and metconazole) were more effective than the strobilurins. The highest efficiency rates were achieved by application of pyraclostrobin + fluxapyroxad, pyraclostrobin + epoxiconazole + fluxapyroxad and fentin hydroxide. Disease severity affected cotton yield, causing losses from 14.8% to 31.7%. Resultant reductions in cotton fiber quality negatively affect the micronaire composition, altering the length, strength and uniformity of fibers.
Highlights
Ramularia leaf spot (RLS) is a devastating disease of cotton caused by the fungus Ramulariopsis gossypii (Speg.) U
Experiments were favorable to cotton development, which together with the cultivars’ susceptibility and the inoculum availability in the areas favored increased RLS severity
There was a significant interaction between fungicide application and experiments with respect to cotton yield only (Table 5)
Summary
Ramularia leaf spot (RLS) is a devastating disease of cotton caused by the fungus Ramulariopsis gossypii (Speg.) U. Disease presents at the end of the cotton cycle and was considered of only secondary importance until the 90’s. With the increase of area cultivated in the Midwest and the increased use of susceptible cultivars, it occurs earlier and it is currently considered the main cotton disease in Brazil (SUASSUNA et al, 2008). The disease causes early defoliation and reduces the photosynthetic capacity of infected plants. Reductions in yield vary depending on the cultivar used, infection time and disease severity. Damages ranging from 30 to 75% can occur in Brazil due to the RLS (SUASSUNA et al, 2008; NOVAES et al, 2011; ZANDONÁ et al, 2012)
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