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Alternaria brown spot (ABS), caused by Alternaria alternata, has been controlled with sprays of quinone outside inhibitors (QoI), demethylation inhibitors (DMI) and copper-based fungicides. QoI fungicides have been intensively used for citrus disease control in Sao Paulo (SP) since 2003, when ABS was first found in the state. In 2017, citrus growers from several regions in SP stated experiencing failures in ABS control with the use of QoI fungicides. This study was then proposed to assess the sensitivity of A. alternata isolates to QoI fungicides by conidial germination and resazurin microtiter assay, and also to determine the geographic distribution of sensitive and resistant isolates in areas cultivated with tangerines and their hybrids in SP. One hundred and twenty-eight A. alternata isolates were sampled in nine municipalities in different SP regions in 2017 and 2018. Isolates collected in 2003/2004, immediately after the first report of ABS in SP, were used as baseline. Effective concentrations of azoxystrobin and pyraclostrobin to reduce conidial germination by 50% (EC50) were estimated and A. alternata isolates were classified as resistant when the conidial germination rate was ≥50% at a discriminatory rate of 1 μg mL−1. Isolates resistant to QoI fungicides were widely found throughout SP regions, and no pattern in resistance distribution was observed. In five out of the nine municipalities, 100% of isolates were resistant to both fungicides, while in other four municipalities the percentage of resistant isolates to azoxystrobin (46%–100%) was higher than that of pyraclostrobin (15%–93%). The G143A mutation positively correlated with the resistance phenotype to azoxystrobin for all A. alternata isolates and to pyraclostrobin for most of them. The resazurin microtiter assay proved useful for rapid qualitative classification of sensitive and resistant A. alternata isolates. Our results suggest the need for anti-resistance strategies in the chemical management of the disease in the state.

Citrus postbloom fruit drop (PFD) caused by Colletotrichum spp. occurs in several countries in the Americas reducing yields by as much as 80%. Fungicide application is the main strategy for PFD control. Two field trials were performed to assess the timing of applications and another two were set up to investigate the efficacy of fungicides for PFD control in Sao Paulo State, Brazil. The percentage of symptomatic flowers per branch, the number of persistent calyces per branch, the number of fruit per branch and yield were evaluated. Four sprays of trifloxystrobin + tebuconazole, carbendazim, difenoconazole or cyprodinil + fludioxonil were effective in reducing yield losses due to PFD, however the trifloxystrobin + tebuconazole mixture was significantly more effective than all other treatments. The conventional PFD control programme, with one spray of difenoconazole followed by up to three carbendazim applications at different intervals, showed variable results, probably due to rainfall. The trifloxystrobin + tebuconazole mixture was effective at controlling PFD under favourable conditions for infection and can be recommended in PFD control programmes.

The incidence and severity of citrus variegated chlorosis (CVC) caused by Xylella fastidiosa are higher in the northern region of Sao Paulo, Brazil than in the southern region. This phenomenon is partly due to differences in the climate, especially rainfall, as the northern region tends to be drier than the southern region. The progress of CVC under three levels of irrigation was assessed in a grove located in the northern region of Sao Paulo. Trees (10-year-old Natal sweet orange) were arranged in a randomized complete block in a 3 × 2 factorial scheme with three levels of irrigation and two methods of infection with X. fastidiosa . The disease incidence in branches and the number of symptomatic fruits per tree were evaluated for three years. A monomolecular model was used to describe the progress of CVC incidence for all treatments. Irrigation reduced CVC symptoms in trees, especially the number of symptomatic fruits per tree. Based on this significant reduction, citrus irrigation can be used to reduce the main negative effect of CVC: reduced fruit size. Our results also explain the higher incidence of CVC in the northern region of Sao Paulo, where a dry season occurs.

Few studies have focused on the effect that silicon fertilisation has on the incidence of brown rust in sugarcane, even though this crop is Si-accumulating and Si addition has been shown to have positive effects in other plant-pathosystems. The objective of this study was to quantify brown rust development in sugarcane after the addition of silicon fertilizer to texturally distinct soils over three sugarcane cropping cycles. One experiment was conducted for three years with sugarcane grown in 100 L pots receiving four levels of silicon fertilizer (0, 185, 370, 555 kg ha −1 Si), with three different soil types (Quartzipsamment-RQ; Rhodic Hapludox-LV; Rhodic Acrudox-LVdf), in randomised blocks design with four replications. Silicate fertilisation increased Si concentrations in the soil and sugarcane leaves. The incidence of brown rust was influenced by Si rates and soils type for the plant cane and second ratoon. For the plant cane, Si application reduced the maximum rust incidence, which was estimated by the b 1 parameter of the monomolecular model, with values reaching 29%, 41% and 47% for the RQ, LV and LVdf, respectively. For the RQ, the b 1 value was reduced by 20%, 20% and 59% of the control treatment values for the 185, 370 and 555 kg ha −1 Si application rates, respectively. There was a significant reduction in brown rust incidence as a function of increasing leaf Si concentrations in all soil types. These results suggest that Si fertilisation could play an important role in the management of brown rust for sugarcane.

Guignardia citricarpa, the causal agent of citrus black spot (CBS), forms ascospores and conidia in citrus leaf litter and only conidia on fruit and twigs. The objective of this work was to determine the relative importance of inoculum sources of ascospores and conidia in the progress of CBS under natural conditions in Brazil. A first experiment was conducted in an orchard subdivided into two areas. In one area, fallen leaves were removed and in the other area fallen leaves were maintained. In each area, two treatments were established: 1) CBS symptomatic mature fruit remained on the trees after the new fruit set, and 2) CBS symptomatic mature fruit were harvested before the new fruit set. A second experiment was carried out in a CBS-free experimental orchard where detached symptomatic fruit and leaf litter on the orchard floor and detached symptomatic fruit and dead twigs on the tree canopy were distributed to investigate the spread of disease. In the first experiment disease incidence was similar for all treatments in all years. However, in 2003, in the area without leaf litter (absence of the ascospores source), disease intensity and the rate of progress of disease incidence were higher when there was overlap of mature and young fruit. In the second experiment, only detached symptomatic fruit and dead twigs on the tree canopy were able to spread CBS in two years of the experiment. The distance of disease spread was less than 80 cm from these inoculum sources. The removal of fallen leaves was not sufficient to completely suppress the disease because of the presence of conidia in fruit and dead twigs. Therefore the reduction of conidia sources should be considered in CBS management in Brazil.

Brown rot, caused by Monilinia fructicola , is the most widespread disease for organic peach production systems in Brazil. The objective of this study was to determine the favorable periods for latent infection by M. fructicola in organic systems. The field experiment was carried out during 2006, 2007 and 2008 using the cultivar Aurora. After thinning fruits were bagged using white paraffin bags, and the treatments were performed by removing the bags and exposing the fruit for four days to the natural infection during each of seven fruit stages from pit hardening to harvest. Throughout the entire growing season, the conidial density and the weather variables were measured and related to the disease incidence using multiple regression analyses. At the fourth day after harvest in each season, the cumulative disease incidence was assessed, and it ranged from 40 to 98%. The incidence of brown rot on fruit that were exposed during the embryo growing stage was lower than that of unbagged fruit throughout the entire season in 2006 and 2008. The relative humidity and the conidia density were significantly correlated to disease incidence. Based on our results, M. fructicola can infect peaches during any stage of fruit development, and control of the disease must be revised to account for organic peach production systems.

Citriculture in Sao Paulo State, Brazil, is threatened by Asiatic citrus canker (Xanthomonas axonopodis pv. citri). The introduction of the Asian citrus leaf miner (Phyllocnistis citrella [CLM]) has resulted in an increase in the number of disease foci and has changed the spatial pattern of citrus canker symptomatic trees from strong aggregation to intermediate aggregation and random patterns. We evaluated the effect of inoculum concentration (101, 102, 104 and 106 colony forming units (cfu) ml−1) on infection of leaves of Tahiti lime (Citrus latifolia) with three distinct treatments: (1) intact leaves, (2) mechanically wounded leaves, and (3) leaves exhibiting injuries from CLM at the egg stage, first instar, third instar, and pupal stages. The minimum inoculum concentration to cause symptom development in intact leaves was 104 cfu ml−1; in mechanically wounded leaves and in leaves with CLM injury at the egg stage and first instar stage, 102 cfu ml−1; in leaves with CLM injuries at the third instar and pupa stage, 101 cfu ml−1. The injuries from the third instar and pupa stages resulted in greater disease severity than other treatments at all inoculum concentrations (five times higher than in the intact leaf). Disease severity in leaves with mechanical wounding and with CLM injuries caused at the egg and first instar stages did not differ from intact leaves.