Lack of an Intron in Cytochrome b and Overexpression of Sterol 14α-Demethylase Indicate a Potential Risk for QoI and DMI Resistance Development in Neophysopella spp. on Grapes.

Asian grapevine leaf rust, caused by Neophysopella meliosmae-myrianthae and N. tropicalis, is often controlled by quinone outside inhibitor (QoI) and demethylation inhibitor (DMI) fungicides in Brazil. Here, we evaluated the sensitivity of 55 Neophysopella spp. isolates to pyraclostrobin (QoI) and tebuconazole (DMI). To elucidate the resistance mechanisms, we analyzed the sequences of the cytochrome b (CYTB) and cytochrome P450 sterol 14α-demethylase (CYP51) target proteins of QoI and DMI fungicides, respectively. The CYP51 expression levels were also determined in a selection of isolates. In leaf disc assays, the mean 50% effective concentration (EC50) value for pyraclostrobin was about 0.040 µg/ml for both species. CYTB sequences were identical among all 55 isolates, which did not contain an intron immediately after codon 143. No amino acid substitution was identified at codons 129, 137, and 143. The mean EC50 value for tebuconazole was 0.62 µg/ml for N. tropicalis and 0.46 µg/ml for N. meliosmae-myrianthae, and no CYP51 sequence variation was identified among isolates of the same species. However, five N. meliosmae-myrianthae isolates grew on leaf discs treated at 10 µg/ml tebuconazole, and these were further exposed to tebuconazole selection pressure. Tebuconazole-adapted laboratory isolates of N. meliosmae-myrianthae showed an eight- to 25-fold increase in resistance after four rounds of selection that was not associated with CYP51 target alterations. In comparison with sensitive isolates, CYP51 expression was induced in the presence of tebuconazole in three out of four tebuconazole-adapted isolates tested. These results suggest a potential risk for QoI and DMI resistance development in Neophysopella spp.

Mycosphaerella musicola Identified as the Only Pathogen of the Sigatoka Disease Complex Present in Minas Gerais State, Brazil.

A thorough assessment of the distribution of Mycosphaerella spp. associated with banana in Minas Gerais State, Brazil, was conducted after Mycosphaerella fijiensis was first reported to occur in this region in 2005. From 2009 to 2011, 80 fields located in 20 municipalities including the same fields where the disease was first reported were sampled. A total of 800 samples of leaf tissue with symptoms similar to those of yellow or black Sigatoka diseases were examined, and 239 isolates were obtained. The identification of the fungi was based on morphological characters combined with DNA sequences obtained after amplification with species-specific primers and phylogeny inferred from the internal transcribed spacer region of Mycosphaerella strains from banana. All 239 isolates were identified as Mycosphaerella musicola. The absence of M. fijiensis in the samples may have been due to misidentification of M. fijiensis or the displacement of M. fijiensis by M. musicola. It is now apparent that yellow Sigatoka caused by M. musicola is the prevailing leaf spot disease of bananas in Minas Gerais State and that regulatory/legislative control measures need to be revised based on our findings.

Comparative transcriptomic provides novel insights into the soybean response to Colletotrichum truncatum infection.

Introduction: Soybean (Glycine max) is among the most important crops in the world, and its production can be threatened by biotic diseases, such as anthracnose. Soybean anthracnose is a seed-borne disease mainly caused by the hemibiotrophic fungus Colletotrichum truncatum. Typical symptoms are pre- and post-emergence damping off and necrotic lesions on cotyledons, petioles, leaves, and pods. Anthracnose symptoms can appear early in the field, causing major losses to soybean production. Material and Methods: In preliminary experiments, we observed that the same soybean cultivar can have a range of susceptibility towards different strains of C. truncatum, while the same C. truncatum strain can cause varying levels of disease severity in different soybean cultivars. To gain a better understanding of the molecular mechanisms regulating the early response of different soybean cultivars to different C. truncatum strains, we performed pathogenicity assays to select two soybean cultivars with significantly different susceptibility to two different C. truncatum strains and analyzed their transcriptome profiles at different time points of interaction (0, 12, 48, and 120 h post-inoculation, hpi). Results and Discussion: The pathogenicity assays showed that the soybean cultivar Gm1 is more resistant to C. truncatum strain 1080, and it is highly susceptible to strain 1059, while cultivar Gm2 shows the opposite behavior. However, if only trivial anthracnose symptoms appeared in the more resistant phenotype (MRP; Gm1-1080; Gm2-1059) upon 120 hpi, in the more susceptible phenotype (MSP; Gm-1059; Gm2- 1080) plants show mild symptoms already at 72 hpi, after which the disease evolved rapidly to severe necrosis and plant death. Interestingly, several genes related to different cellular responses of the plant immune system (pathogen recognition, signaling events, transcriptional reprogramming, and defense-related genes) were commonly modulated at the same time points only in both MRP. The list of differentially expressed genes (DEGs) specific to the more resistant combinations and related to different cellular responses of the plant immune system may shed light on the important host defense pathways against soybean anthracnose.