Spot blotch, caused by Cochliobolus sativus (Ito & Kuribayashi) Drechs. ex Dastur, is one of the important diseases of wheat worldwide. The main objective of this study was to investigate the phenotypic and genotypic variability among C. sativus isolates from the hills and plains in Nepal. A total of 48 monoconidial isolates of C. sativus from the hills (n0 24 isolates) and plains (n024 isolates) in Nepal were analyzed for morphology, aggressiveness and genetic structure. C. sativus isolates were grouped into three categories on the basis of their colony texture and mycelia colour. Thirteen isolates from the hills and plains belonging to three morphological groups were randomly selected and evaluated for aggressiveness on eight wheat cultivars (Chirya 1, Chirya 7, Milan/ Shanghai 7, SW 89-5422, PBW 343, BL 1473, BL 3036, and RR 21) at the seedling stage. Nonparametric analysis revealed that the isolates from the plains (median disease rating of 5) were significantly (P0 0.0001) more aggressive than the isolates from the hills (median disease rating of 3). A significant (P0 0.0001) isolate by cultivar interaction was demonstrated and the isolates from the same geographic region and morphological group displayed different degrees of aggressiveness on wheat cultivars tested. Combined IS-PCR and rep-PCR analyses revealed moderate gene diversity (H00.24 and 0.25 for the hills and plains, respectively). Low linkage disequilibrium (LD) value and non-significant (P00.001) population differentiation (G″ ST 00.05) were detected, indicating that isolates of C. sativus from the hills and plains in Nepal were genetically similar. Analysis of molecular variation (AMOVA) revealed low (7%) levels of genetic variation between the hill and plain populations, whereas >93% of genetic variation was found within populations. Overall, C. sativus isolates from Nepal are pathologically and genetically diverse, and such information will be useful in developing wheat cultivars resistant to C. sativus.
In 2009, a blackleg outbreak was detected in canola fields of North Dakota. The disease, which is caused primarily by the fungus Leptosphaeria maculans, was observed in 88 of 99 fields scouted with 39 fields having incidences >30%. The mean blackleg incidence in these 39 fields was 61% (range 50 to 84%). Fifty nine L. maculans isolates were retrieved from 20 of these 39 fields and classified in five pathogenicity groups (PG) using a set of three differentials cultivars. PG-4 was the pathotype most commonly isolated from these fields with 51% of all isolates, followed by PG-3 with 25% and PG-T with 8% of isolates. Only 3% of the isolates belong to PG-2, a pathotype that was previously considered the most prevalent in the region. Increased prevalence of these new pathogenicity groups represents a threat to the canola industry in the state. Accepted for publication 20 January 2012. Published 10 April 2012.
Blackleg, caused by Leptosphaeria maculans, was first reported on canola (Brassica napus) in North Dakota in 1991. In 2003, L. maculans strains of previously unreported pathogenicity groups (PG) were discovered in the region. Since then, however, little has been known about the prevalence of L. maculans in the state. The objectives of this study, therefore, were to characterize the prevalence of blackleg and of L. maculans PGs in North Dakota. Prevalence was assessed in 2004, 2007, and 2009 in 572 fields. PG determination for 216 L. maculans isolates retrieved from blackleg symptomatic stems during that period was achieved on a set of B. napus differential cultivars. Blackleg prevalence increased from 28% in 2004 to 63 and 74% in 2007 and 2009, respectively. Similarly, the number of fields with blackleg incidences >30% increased from 4% in 2004 to 12 and 23% in 2007 and 2009, respectively. In all years, PG-4 was the predominant group, while PG-2, once predominant, accounted for <2% of isolates. Increase in the prevalence and incidence of blackleg as well as the frequency of virulent PGs over the last 10 years is a serious threat to the canola industry of the region.
Nepal, A., and del Rio Mendoza, L. E. 2012. Effect of sclerotial water content on carpogenic germinafion of Sclerotinia sclerotiorum. Plant Dis. 96:1315-1322.
Winter canola (Brassica napus L.) is a relatively new crop to Oklahoma and other southern U.S. states where it is considered a desirable rotation crop with wheat. In 2009, approximately 15,000 ha of winter canola were harvested in Oklahoma (3); that area is expected to almost double in 2010. Blackleg, a potentially devastating canola disease, was detected in Oklahoma in 2009. Blackleg is caused by Leptosphaeria maculans (Desmaz.) Ces. & de Not (anamorph = Phoma lingam (Tode:Fr.) Desmaz.). In early 2010, leaf samples showing typical symptoms of blackleg were collected from four canola fields near the town of Enid in Garfield County, OK. Small portions of infected tissues were surface disinfested in an aqueous solution of NaOCl (0.5% a.i.) for 1 min, rinsed twice in sterile distilled water, and plated on V8 medium. Seven colonies were isolated and when grown in pure culture, all produced 2 × 4.5 μm guttulate, unicellular, hyaline spores in pycnidia that ranged from 200 to 480 μm in diameter. These morphological characteristics correspond with those of P. lingam (1). To verify the pathogenic nature of the isolates and establish the pathogenicity group (PG) to which they belong, a standard inoculation protocol was followed on a set of three differential cultivars, Quinta, Glacier, and Westar (2). Briefly, for each isolate, tiny puncture wounds were made with sterile needles on the cotyledons of six 10-day-old plants of each differential and a 10-μl aliquot of a pycnidiospore suspension (1 × 107 spores ml–1) was deposited on the wounds. Also, a set of differentials were inoculated with distilled water (mock inoculation). Inoculated plants were incubated overnight in a misting chamber at 21°C in the dark and returned the next day to the greenhouse. Disease severity was recorded 10 days after inoculation using a 0 to 9 scale in which 0 to 2 = resistant, 3 to 6 = intermediate, and 7 to 9 = susceptible. This process was repeated three times. Two of the seven isolates evaluated were highly virulent on all three differentials, an indication they belong to pathogenicity group 4 (2). The other five isolates produced small lesions on Westar (resistant reaction) but failed to develop symptoms on the other two differentials. This phenotypic reaction has been associated with strains of PG-1. Mock-inoculated plants did not develop lesions. To our knowledge, this is the first time blackleg isolates from Oklahoma have been identified to the PG level. While this information will assist breeders in the development of both spring and winter canola lines with resistance to blackleg, additional studies are necessary to determine the relative prevalence and diversity of the various PG in Oklahoma. References: (1) G. H. Boerema. Trans. Br. Mycol. Soc. 67:289, 1976. (2) A. Mengistu et al. Plant Dis. 75:1279, 1991. (3) USDA. National Agricultural Statistics Service. Retrieved from http://www.nass.usda.gov/Statistics_by_State/Ag_Overview/ AgOverview_OK.pdf , September 20, 2010.
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