Effect of pathogen concentrations and period of leaf wetness on orange rust severity in Brazilian sugarcane cultivars

Abstract

The most important management strategy for orange rust of sugarcane (Puccinia kuehnii) is the use of resistant cultivars. Nevertheless, canes with intermediate resistance are still grown over large areas in the Brazilian sugarcane industry, with varying degrees of disease severity. Factors governing the severity of orange rust in these crops are largely unknown. Therefore, the present work aimed to examine the influence of pathogen concentration, period of leaf wetness, and urediniopore survival on sugarcane residue on disease severity. Thirty-day-old plants of cultivars RB72454, RB855156, RB867515, RB92579, RB935744, RB975201, SP81–3250, and SP89–1115 were inoculated with 50 ml of spore suspension with varying urediniospore concentrations (from 1.10¹ to 10⁷ spores.ml⁻¹) or period of leaf wetness (12, 24, and 48 h, pathogen concentration 1.10⁶ spores.ml⁻¹). Reaction of cultivars was evaluated by assessing leaf area affected. Survival of urediniospores was examined on sugarcane residues at 0, 10, 15, 20 days after separation from the cane plant. Only RB72454, RB92579, and RB855156, classified as susceptible or intermediate, exhibited higher disease levels as pathogen concentration increased. Extended periods of leaf wetness only influenced susceptible/intermediate cultivars, where higher severity occurred with longer period of leaf wetness. Viability of urediniospores collected in winter always had a greater loss of viability compared to spring but both season harbored viable urediniospores on 20-day-old sugarcane residues, suggesting they potentially could play a role in initiation of disease epidemics.