Microclimate and ramulosis occurrence in a cotton crop under three plant population densities in Southern Brazil

This study aimed to evaluate crop microclimate and its influence on ramulosis disease in a cotton crop conducted in three population densities. The experiment was carried out in Piracicaba, state of São Paulo, Brazil, where the genotypes IAC 23 and Coodetec 401 were sowed with the following plant population densities: 55,000; 111,000; and 166,000 plants per hectare. To start the epidemic process, a conidia suspension of Colletotrichum gossypii var. cephalosporioides was inoculated in the crop at 30 and 45 days after sowing. The weather variables, air temperature, relative humidity and leaf wetness duration, were recorded with an automatic weather station located at the experimental area and with six micro-stations located within the crop canopy (three in each genotype). Results showed that plant density had low effect on air temperature, but differences were found on relative humidity and leaf wetness duration. These differences were observed until the canopy became a continuous surface. The microclimate promoted by plant densities showed to have little influence on disease progress, since macroclimate during the experiment was favorable to disease development. Genotype IAC 23 was more resistant to ramulosis than Coodetec 401. The area under disease progress curve presented a define relationship with yield (R² = 0.70 for all treatments and R²sup = 0.93 for averages), being a potential parameter for evaluating the impact of ramulosis disease on cotton production in southern Brazil.