Quinone-outside inhibitor (QoI) fungicides are effective tools for preharvest control of brown rot of stone fruit. These fungicides have a very specific site of action so the risk of resistance selection is high. The sensitivity of Monilinia fructicola (G. Winter) Honey isolates to azoxystrobin (QoI) was investigated in 143 isolates collected between 2002 and 2011 from four Brazilian states in orchards with different frequencies of fungicide use (0 to 6 fungicides sprays/season). Sensitivity of the isolates to azoxystrobin was determined in vitro, by inhibition of mycelial growth and spore germination on fungicide-amended media or ex vivo by pathogen inoculation in untreated or treated fruit with azoxystrobin. Potential mutations in codons 143, 137, and 129 of the cytochrome b (Cyt b) gene and the occurrence of an intron immediately after codon 143 were analyzed in a subpopulation of the isolates. The M. fructicola population of São Paulo State was less sensitive to the fungicide than the population from the states of Paraná, Santa Catarina, and Rio Grande do Sul. The low sensitivity of the isolates was confirmed also by comparing to the sensitivity of the baseline isolates. Mutations in G143A, F129L, and G137R in Cyt b gene were not found. In addition, 58 isolates tested showed an intron after codon 143 in Cyt b gene. Our results indicate that other mechanisms of selection for low sensitivity to QoI fungicides should be investigated.

The management of citrus canker, caused by Xanthomonas citri subsp. citri, has been widely studied in endemic areas because of the importance of the disease in several citrus-producing countries. A set of control measures is well established, but no study has investigated the efficiency of each measure individually and their combination for disease suppression. This study comprised a 3-year field study to assess the relative contribution of three measures for the control of citrus canker and reduction of crop losses. Windbreak (Wb), copper sprays (Cu), and leafminer control (Lc) were assessed in eight different combinations in a split−split plot design. The orchard was composed of ‘Valencia’ sweet orange trees grafted onto ‘Rangpur’ lime. Casuarina cunninghamiana trees were used as Wb. Cu and Lc sprays were performed every 21 days throughout the year. Individually, Cu showed the highest contribution for canker control, followed by Wb. Lc had no effect on reducing citrus canker. Wb+Cu showed the highest efficiency for control of the disease. This combination reduced the incidence of diseased trees by approximately 60%, and the incidence of diseased leaves and fruit by ≥90% and increased the yield in 2.0- to 2.6-fold in comparison with the unmanaged plots. Cu sprays were important for reducing disease incidence and crop losses, whereas Wb had an additional contribution in minimizing the incidence of cankered, non-marketable fruit. The results indicated that the adoption of these measures of control may depend on the characteristics of the orchard and destination of the production.

Although the implementation of area-wide inoculum reduction and Asian citrus psyllid control are decisive in controlling Huanglongbing (HLB) epidemics, growers continue looking for enhanced nutritional and/or resistance inducer (ENRI) treatments to mitigate yield damage in HLB-affected trees. Therefore, an experiment was set up in an 8-year-old sweet orange orchard to evaluate the effects of the foliar application of nutrients, phosphite, and salicylate sprayed four to five times per year during flushing periods. The trial had eight treatments in four randomized blocks. HLB incidence and yield were assessed from December 2010 to January 2017. HLB-symptomatic trees with less than 5% HLB severity were selected from each plot in the first (Group 1), second (Group 2), and third years (Group 3) after the beginning of treatments to assess the frequency of trees with different HLB symptoms and micronutrient deficiencies, HLB severity, and fruit yield until January 2017. Twenty asymptomatic trees from each plot were also harvested for comparison each year. ENRI treatments did not reduce the progress of HLB incidence in the plots or the HLB severity progress or frequency of HLB symptoms in the selected trees. ENRI treatments did not improve the yield of symptomatic trees compared with asymptomatic trees. Hence, in a scenario without HLB-symptomatic tree removal in mature orchards, disease severity progress and yield reduction in HLB-affected trees remained regardless of ENRI treatment. [ ] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .