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Sclerotinia stem rot (Sclerotinia sclerotiorum) is a serious disease in oilseed Brassica crops world-wide. Temperature adaptation in isolates of S. sclerotiorum collected from differing climatic zones is reported for the first time on any crop. S. sclerotiorum isolates from oilseed rape (B. napus) crops in warmer northern (WW3, UWA 7S3) differed in their reaction to temperature from those from cooler southern (MBRS-1, UWA 10S2) agricultural regions of Accepted ArticleThis article is protected by copyright. All rights reserved.Western Australia in virulence on B. carinata, growth on agar, and oxalic acid production.Increasing temperature from 22/18ºC to 28/24ºC increased lesion diameter on cotyledons of B. carinata BC05411344 more than ten-fold for warmer region isolates, but did not affect lesion size for cooler region isolates. Mean lesion length averaged across two B. carinata genotypes (resistant and susceptible) fell from 4.6 mm to 2.4 mm for MBRS-1 when temperature increased from 25/21ºC to 28/24ºC but rose for WW3 (2.35 mm and 3.21 mm, respectively). WW3, usually designated as low in virulence, caused as much disease on stems at 28/24ºC as MBRS-1, historically designated as highly virulent. Isolates collected from cooler areas grew better at low temperatures on agar. While all grew on potato dextrose agar between 5-30ºC, with maximum growth at 20-25ºC, growth was severely restricted above 32ºC, and only UWA 7S3 grew at 35ºC. Oxalate production increased as temperature increased from 10 to 25⁰C for isolates MBRS-1, WW3 and UWA 7S3, but declined from a maximum level of 101 mg/g mycelium at 20⁰C to 24 mg/g mycelium at 25⁰C for UWA 10S2.

Powdery mildew (Erysiphe cruciferarum) is an important disease in oilseed rape crops worldwide, but of sporadic importance in most southern Australian crops. Six Brassica napus cultivars were exposed to E. cruciferarum simultaneously in four plant age cohorts. First symptoms of powdery mildew appeared 9 days after inoculation (dai) on the oldest plants [42 days after seeding (das)], but 44 dai in the youngest plants that were exposed to inoculum from sowing, although final disease severity did not differ with the plant age at exposure. The maximum level of pod peduncle infestation was unaffected by plant age (P = 0.37) or cultivar (P = 0.28). The effect of temperature was also investigated. The development of disease on plants was slower and final severity reduced at a day/night temperature 14/10 °C compared with 22/17 °C. In vitro, maximum growth of germ tubes from conidia of E. cruciferarum was at 15–20 °C and survival of conidia reduced by temperatures >30 °C. The results explain the sporadic nature of powdery mildew outbreaks in winter‐grown oilseed rape in Australia, where slow rates of infection occur when seasonal colder prevailing winter conditions coincide with the presence of younger plants, together curtailing rapid disease development until temperatures increase in late winter/early spring. These results explain why epidemics are most severe in the two warmer cropping regions, viz. the northern agricultural region of Western Australia and New South Wales. This study suggests that with increases in winter temperatures under future climate scenarios, earlier and more severe powdery mildew outbreaks in Australia will be favoured.

A variety of mechanisms contribute to host resistance against S. sclerotiorum across the three Brassica species. These complex interactions between pathogen and host help to explain variable expressions of resistance often observed in the field.

Commercial canola crops were surveyed in Western Australia during 2008-2010 growing seasons to determine the occurrence of various diseases in canola production. A total of 99, 74 and 53 samples were collected during 2008, 2009 and 2010 respectively. About 100 stems were collected for each sample along a 200m transect. Stems were washed and rated for incidence and severity of internal infection of blackleg. Plants were also assessed for the incidence of the diseases Sclerotinia stem rot caused by Sclerotinia sclerotiorum, club root caused by Plasmodiophora brassicae (from samples collected from northern areas only) and powdery mildew caused by Erysiphe cruciferarum. All four fungal diseases (Blackleg, Sclerotinia, club root and powdery mildew) were detected during the 2008 survey while in 2009 and 2010 surveys, only three main diseases (Blackleg, Sclerotinia and club root) were detected. Prevalence of blackleg was highest (100%) followed by that of Sclerotinia stem rot, and club root respectively in all three years of survey. Averaged across all samples, the mean percent disease index (internal infection) of blackleg was 27, 48 and 28% and the mean incidence of Sclerotinia stem rot was 11, 9 and 3% during 2008, 2009 and 2010 respectively. Likewise, the average incidence of club root from northern areas was 11, 25 and 7% during 2008, 2009 and 2010 respectively. Seasonal conditions, particularly the dry conditions experienced in 2010, were significant modifiers of the incidence and severity of diseases observed in this survey.

Field resistances against Sclerotinia rot (SR) (Sclerotinia sclerotiorum) were determined in 52 Chinese genotypes of Brassica oleracea var. capitata, 14 Indian Brassica juncea genotypes carrying wild weedy Brassicaceae introgression(s) and four carrying B-genome introgression, 22 Australian commercial Brassica napus varieties, and 12 B. napus and B. juncea genotypes of known resistance. All plants were individually inoculated by securing an agar disc from a culture of S. sclerotiorum growing on a glucose-rich medium to the stem above the second internode with Parafilm tape. Mean stem lesion length across tested genotypes ranged from <1 to >68 mm. While there was considerable diversity within the germplasm sets from each country, overall, 65% of the B. oleracea var. capitata genotypes from China showed the highest levels of stem resistance, a level comparable with the highest resistance ever recorded for oilseed B. napus or B. juncea from China or Australia. One Indian B. juncea line carrying weedy introgression displayed a significant level of both stem and leaf resistance. However, the vast majority of commercial Australian oilseed B. napus varieties fell within the most susceptible 40% of genotypes tested for stem disease. There was no correlation between expressions of stem versus leaf resistance, suggesting their independent inheritance. A few Chinese B. oleracea var. capitata genotypes that expressed combined extremely highlevel stem (≤1 mm length) and leaf (≤0.5 mean number of infections/ plant) resistance will be particularly significant for developing new SRresistant horticultural and oilseed Brassica varieties. Zhao et al. 2006;Delourme et al. 2011;Disi et al. 2014). However, consistent effort over multiple years has revealed high levels of resistance in a

Sclerotinia stem rot, caused by Sclerotinia sclerotiorum, is a serious disease of many cruciferous crops and frequently poses a threat to the sustainable and profitable production of these crops worldwide. Differences in seedling resistance to S. sclerotiorum across 46 diverse cruciferous genotypes from 12 different species were assessed by comparing the extent of pathogenesis on inoculated cotyledons under controlled conditions. Selections of Brassica carinata, B. incana, B. juncea, B. napus, and B. napus introgressed with B. carinata, B. nigra, B. oleracea, B. rapa var. rosularis, B. rapa var. chinensis, B. tournefortii, Raphanus raphanistrum, R. sativus, and Sinapis arvensis were tested. The average size of lesions on cotyledons 48 h post inoculation varied from 0.8 to 7.3 mm. The three most resistant genotypes with the smallest lesions were all from B. oleracea (viz., B. oleracea var. italica ‘Prophet’ and B. oleracea var. capitata ‘Burton’ and ‘Beverly Hills’). Representatives of R. raphanistrum, S. arvensis, B. juncea, and B. carinata were the most susceptible to S. sclerotiorum, with the largest lesions. To our knowledge, this is the first report of high levels of resistance to S. sclerotiorum in B. oleracea at the cotyledon stage and also the first report of the host cotyledon reactions against S. sclerotiorum for all tested species except B. napus and B. juncea. The mean lesion size for B. napus introgressed with B. carinata was 5.6 mm, which is midway between the lesion size for the two parent species B. napus (5.1 mm) and B. carinata (5.8 mm). Separate genetic control for cotyledon versus mature plant resistance was demonstrated by the lack of correlation between lesion size from S. sclerotiorum on the cotyledon with the severity of disease initiated by stem inoculation or natural processes in a previous field test. On the most resistant genotypes, B. oleracea var. italica Prophet and var. capitata Burton, growth of S. sclerotiorum on the cotyledon surface prior to penetration was severely impeded, production of appressoria inhibited, and both cytoplasm shrinkage and protoplast extrusion in S. sclerotiorum hyphae prevalent. This is the first report of such resistant mechanisms in B. oleracea. Genotypes with cotyledon resistance identified in this study will be of great value not only in furthering our understanding of resistance mechanisms across different cruciferous species but also could be exploited for developing commercial crucifer cultivars with high-level resistance against S. sclerotiorum.

Summary of recommended strategies:• Use good quality seed that is free of sclerotia.• Avoid sowing canola next to paddocks that were infected with Sclerotinia in the previous three years.• Check for Sclerotinia symptoms in broadleaf crops if considering sowing canola into the same paddock the following year.• Preventative strategic sprays of foliar fungicides at early to mid flowering. Fungicides should only be considered for very high yielding crops in districts prone to Sclerotinia.