Wheat blast caused by Magnaporthe oryzae is a relatively new disease that has caused considerable losses in wheat fields of several South American countries, including Brazil. The 2016 report of wheat blast occurrence in Bangladesh raised concern in South Asia where wheat represents a significant crop. The sources of primary inoculum and survival from season to season of the fungus remain largely unknown. The effect of wheat residues on the onset of blast epidemics and the potential for survival of M. oryzae in the residues were studied under subtropical climatic conditions, in the South of Brazil. The objective of this study was to monitor the saprotrophic development of M. oryzae on wheat debris and explore the relative importance of crop residues as a source of inoculum. The wheat cultivars BRS 229 and Anahuac 75, moderately and highly susceptible to the disease, respectively, were inoculated with a spore suspension of 10 −5 conidia mL −1 using an aggressive (Py 12.1.209) and a less aggressive (Py 12.1.132) isolate. At maturity, a portion of leaves, stems and spikes were detached from plants, and a group of ten lesions were randomly selected and marked on each type of plant organ. The air-dried plant organs were placed separately inside bags and exposed outside. The experiment was conducted over three different time intervals. Each 14 days, samples were taken from the field and tested for sporulation. The survival of the blast fungus decreased rapidly on the rachis when compared to stems and leaves. Sporulation of the fungus was observed on the wheat residues for up to five months. Based on the results of this study, the possibility that the causal agent of wheat blast survives under Brazilian conditions from one crop to another in wheat residues is very low. The management of crop residues is not a key point to control the development of wheat blast. A strong emphasis should be placed on the presence of other hosts.

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The first report of wheat blast in the world was in Brazil, in 1986. Since then, a great effort has been made towards the development of wheat cultivars resistant to this disease, which is caused by the fungus Pyricularia oryzae Triticum (PoT). The objective of this research was to (i) evaluate the resistance of wheat genotypes to blast and (ii) verify the correlation between disease severity on wheat spikes and sporulation rate of PoT on spike rachises. Plants of 40 cultivars grown in pots, at the flowering stage (stage 65 on the Zadoks scale), were inoculated with a suspension of conidia of a PoT isolate representative of the main variant of the fungus reported in Brazil. Severity of blast on the spikes at 5 and 7 days after inoculation (dai) and the rate of sporulation of the fungus on the rachis (conidia per g of rachis) were evaluated. Eighty percent of the cultivars that were classified in the group with the lowest sporulation rate were also classified in the group with the highest resistance at 7 dai. However, the correlation coefficients of the analysis established between the cultivar severity at 5 and 7 dai averages and the PoT sporulation rate averages were not significant (r=0.2464 and r=0.2047, respectively). Results obtained represent the updated characterization to blast of wheat cultivars in Brazil and constitute an important exploratory framework for the evaluation of the reaction of wheat genotypes based on the sporulation rate of PoT on their tissues.

A survey of the relative incidence of anastomosis groups (AG) of Rhizoctonia spp. associated with potato disease was conducted in Idaho, the leading potato producing state in the USA. In total, 169 isolates of Rhizoctonia solani and seven binucleate Rhizoctonia (BNR) isolates were recovered from diseased potato plants. The AG of each isolate was determined through real-time PCR assays for AG 3-PT and phylogenetic analysis of the internal transcribed spacer region of ribosomal DNA. AG 3-PT was the predominant AG accounting for 85% of isolates recovered, followed by AG 2-1 (5.7%) and AG 4 HG-II (4.5%). Two different subsets of AG 2-1 isolates were recovered (subset 2 and 3). Three isolates each of AG A and AG K were recovered, as well as one isolate each of AG 5 and AG W. An experiment carried out under greenhouse conditions with representative isolates of the different AGs recovered from Idaho potatoes showed differences in aggressiveness between AGs to potato stems, with AG 3-PT being the most aggressive followed by an isolate of AG 2-1 (subset 3). The three BNR isolates representative of AG A, AG K and AG W appeared to be less aggressive to potato stems than the R. solani isolates except for the AG 2- 1 (subset 2) isolate. This is the first comprehensive study of the relative incidences of Rhizoctonia species associated with Idaho potatoes and the first study to report the presence of BNR AG W outside of China.

In September of 2018, onion plants (Allium cepa cv. Joaquin) grown in one field in southwest Idaho were observed to have roots with brown discoloration over 10-20% of the total root surface area. Approximately 10% of plants over a 1 ha area were affected and these plants were about visually 50% smaller than the typical bulb size present in the field. To determine the causal agent, 3 mm pieces of symptomatic roots from four plants were placed in sodium hypochlorite (2%) for one minute, followed by two rinses in sterile water and plated on to water agar medium amended with penicillin G (0.2 g/liter) and streptomycin sulfate (0.8 g/liter). After 3 days at 21°C, fungal colonies with septate hyphae with right-angled branching resembling Rhizoctonia solani were observed in over half of the 16 isolations attempted. Species identity was confirmed through rDNA ITS sequencing, as described previously (Woodhall et al., 2013), with DNA obtained from a single representative hyphal tip culture grown on Potato Dextrose Agar (PDA) which was designated isolate ON3. The resulting sequence (MT672318), was 100% identical (678/678bp) to a sequence previously identified as R. solani AG 2-2 IIIB on GenBank (FJ492137). Pathogenicity of the culture was determined by inoculating ten 20-day-old plants (cv. Joaquin) grown in premium potting compost (Scotts) with a single, fully colonized 10 mm2 plug taken from a 2-week-old PDA culture of isolate ON3. A further nine plants were inoculated with sterile PDA plugs as controls. Plants were grown in the greenhouse at 21C in a 16-hour light regime. After 24 days, each plant was assessed for root rot disease as described previously (Misawa et al. 2017). Root rot was observed on nine of the inoculated plants. Mean diseased root area was 32% of the total root surface, with a minimum of 5% and a maximum of 100% diseased root area and a standard deviation equal to 39.6. No root browning was observed on any of the control plants. Isolations were attempted from nine symptomatic plants and R. solani was successfully isolated from seven plant samples onto water agar. Sequencing was used to confirm identity as AG2-2IIIB. To our knowledge, this is the first report of R. solani AG 2-2 IIIB affecting onions in Idaho. Previous work in the Pacific Northwest recovered R. solani AG2-1, 3, 4 and 8 and also BNR AG A from stunted onions (Patzek et al., 2013). In Japan, Misawa et al. (2017) found AG 2-2 IIIB to be pathogenic to Welsh onion (Allium fistulosum). In Idaho, R. solani AG 2-2 IIIB has was previously reported causing disease in sugar beets (Strausbaugh et al. 2011) and potatoes (Woodhall et al. 2012). Growers should consider crop rotation strategies or soil treatments if R. solani AG2-2IIIB is causing disease in their crops.