H.C. Huang

Research on biology and control of Sclerotinia diseases in Canada1

Sclerotinia sclerotiorum (Lib.) de Bary and Sclerotinia minor Jagger are two of the species in the genus Sclerotinia found in Canada. While S. sclerotiorum is distributed country-wide and causes disease on numerous economically important crops including oilseeds, pulses, forage legumes, vegetables, and ornamentals, S. minor has only been reported on lettuce in Quebec and Ontario. During the past five decades, Canadian researchers have made a substantial contribution to the understanding of the biology and control of S. sclerotiorum and, to a lesser extent, of S. minor. This review summarizes the progress of Canadian research on Sclerotinia diseases in the past 50 years, with specific emphasis in the areas such as strain differentiation, epidemiology, new types of sclerotia, survival and germination of sclerotia, and biocontrol. Le Sclerotinia sclerotiorum (Lib.) de Bary et le Sclerotinia minor Jagger sont deux espèces du genre Sclerotinia trouvées au Canada. Alors que le S. sclerotiorum a une distribution pancanadienne et cause des maladies sur de nombreuses cultures dimportance économique, y compris des plantes oléagineuses, des légumineuses alimentaires, des légumineuses fourragères, des légumes et des plantes ornementales, le S. minor na été rapporté que sur la laitue au Québec et en Ontario. Au cours des cinq dernières décennies, les chercheurs canadiens ont contribué de façon substantielle à la compréhension de la biologie et de la lutte contre le S. sclerotiorum et, dans une moindre mesure, contre le S. minor. La présente synthèse résume les progrès accomplis par la recherche canadienne sur les sclérotinioses, au cours des derniers 50 ans, en mettant spécifiquement laccent sur des domaines tel que la différenciation des souches, lépidémiologie, les nouveaux types de sclérotes, la survie et la germination des sclérotes et la lutte biologique.

Antagonism and biocontrol potential of Ulocladium atrum on Sclerotinia sclerotiorum

Abstract Antagonism and biocontrol potential of Ulocladium atrum against Sclerotinia sclerotiorum were investigated and compared to another biocontrol agent Coniothyrium minitans of S. sclerotiorum. Dual cultural assay on potato dextrose agar revealed that U. atrum was antagonistic to S. sclerotiorum. It could grow over the colony of S. sclerotiorum, but could not parasitize sclerotia of this pathogen. Results of spore germination tests on PDA and water agar (WA) showed that conidia of U. atrum or pycnidiospores of C. minitans had no inhibitory effects on the germination of ascospores of S. sclerotiorum. Conidia of U. atrum germinated more rapidly and germ tubes of U. atrum developed more extensively than ascospores of S. sclerotiorum and pycnidiospores of C. minitans. When petals of canola (cv Westar) were inoculated with spores of U. atrum or C. minitans at 5.4×104 spores/petal (at 24xa0h before, simultaneously, or 24xa0h after) and ascospores of S. sclerotiorum at 5.0×103 ascospores/petal and placed on detached leaves of canola, incidence of petal-mediated infection of leaves by S. sclerotiorum was significantly reduced. Moreover, the suppression of colonization of petals by S. sclerotiorum was related to the spore concentrations of these two biocontrol agents. Another test on alfalfa showed that application of U. atrum or C. minitans on senescent petals which remained attached to pods of alfalfa significantly reduced the infection of S. sclerotiorum on pods and seeds of alfalfa. This study suggests that the antagonist U. atrum is an effective biocontrol agent of S. sclerotiorum and the efficacy is similar to that of the mycoparasitic fungus C. minitans.

Effect of ground Brassica seed meal on control of Rhizoctonia damping-off of cabbage

Ten cultivars of three Brassica species were compared for volatile compounds in hydrated ground seeds that were fungicidal to Rhizoctonia solani AG-4. Among the samples tested, the volatile substances in the ground seed of mustard (Brassica juncea cv. Bau Sin) showed the strongest fungicidal effect. Allyl isothiocyanate was one of the predominant fungicidal compounds detected in the ground seed of all Brassica species tested, and the amount was the highest in ground mustard seed meals. Among the nine carriers tested for seed coating with ground mustard seed meal, Biolan peat B3 was the most effective for the seed treatment. Cabbage seeds treated with a mixture of ground mustard seed meal and Biolan peat B3 (MBF mixture) at 2:3 ratio (w/w) resulted in a significant reduction in incidence of Rhizoctonia damping-off with no harmful effects to the germination of treated seeds. Effective control of Rhizoctonia damping-off of cabbage by the seed treatment with the MBF mixture was also obtained in a large-scale experiment conducted in a commercial automated environment-controlled greenhouse. This study suggests that the MBF mixture has potential for use in the commercial seed treatment.

Organic amendment of commercial culture media for improving control of Rhizoctonia damping-off of cabbage

Rhizoctonia solani AG-4 is an important damping-off pathogen of cabbage. Using a baiting technique, R. solani was recovered from the cabbage seeds buried in nine commercial culture media. The frequency of seed colonization by R. solani varied with medium, ranging from 2.5% in Nev-3 to 30% in Floradur A and SI 2; the seeds were examined after being buried for 48 h. Among the five formulated organic compounds tested as amendment for the commercial BVB No. 4 medium, FBN-5A was most effective in controlling damping-off. Although both rates of FBN- 5A, 0.1% and 0.2%, significantly reduced incidence of damping-off, FBN-5A at 0.1% was the more suitable treatment because it not only reduced the disease incidence, but also enhanced the growth of cabbage. The results were confirmed in a large-scale experiment in a commercial automatic environment-controlled greenhouse. This study suggests that the FBN-5A compound offers potential for commercial use as an organic amendment for the management of Rhizoctonia damping-o...

Control of damping-off of safflower by bacterial seed treatment

Among 665 strains of rhizosphere bacteria isolated from plant samples in southern Alberta, 23 strains were antagonistic to Pythium sp. group G, a sterile form of Pythium ultimum causing damping-off of safflower. Fifteen of these antagonistic strains were identified at the species level and tested for efficacy as seed-treatments for safflower in soil naturally infested with P. ultimum var. ultimum. Erwinia carotovora, strains 6-7, 6-9-3, and 18-6; E. rhapontici, strains 16-5, 16-7, 17-8, and 18-7; Pseudomonas putida, strain 15-2-3; and P. fluorescens, strains 1-2, 3-3, 6-9-2, and 7-3, significantly (P < 0.05) reduced pre-emergence damping-off and increased seedling emergence in safflower. Bacillus polymyxa strain 5-8; B. subtilis, strain 16-8; and Erwinia herbicola, strain 2-2 were ineffective. Seed treatment with B. polymyxa, strain 5-8; E. rhapontici, strains 17-8 and 18-7; and P. fluorescens, strains 7-3 and 6-9-2 increased seedling height of safflower. The potential of antagonistic strains of rhizosphere bacteria as biocontrol agents for pythium damping-off of safflower is discussed.

Control, by microbial seed treatment, of dampingoff caused by Pythium sp. on canola, safflower, dry pea, and sugar beet

Indoor and field experiments were conducted to determine the effects of seed treatment with strains of rhizosphere bacteria collected from southern and central Alberta for control of damping-off of sugar beet, canola, safflower, and dry pea caused by Pythium sp. Indoor experiments showed that 12 strains of rhizobacteria, including Pseudomonas fluorescens (708, 1-2, 1105, 1809, 2106, and 2202), Bacillus cereus PS1, Bacillus megaterium SB6, Arthrobacter sp. 2101, Pantoea agglomerans (909, 2-2), and Erwinia rhapontici A123, were effective in controlling damping-off of sugar beet. These strains varied in their ability to suppress mycelial growth of Pythium sp. group G, in vitro, and in their ability to secrete extracellular protease. Strains of Pseudomonas fluorescens (708, 1-2, 2202), B. cereus PS1, E. rhapontici A123, and Pantoea agglomerans 2-2 were effective seed-treatment agents for control of damping-off of canola, safflower, dry pea, and sugar beet in fields naturally infested with Pythium spp., although there were some differences in efficacy among the strains for each of the crops. Seed treatment with combination of Pseudomonas fluorescens 708, B. cereus PS1, or E. rhapontici A123 and the fungicide Thiram™ did not improve dry pea emergence compared with the fungicide control under field conditions.

Verticillium wilt of alfalfa: epidemiology and control strategies

Verticillium wilt caused by Verticillium albo-atrum is an important disease of alfalfa (Medicago sativa) in Europe, North America, and northern Japan. The pathogen can spread by various means, including on infected alfalfa seeds and debris, through contact between roots, on farm implements, and through air and water. Recent studies in Canada reveal that V. albo-atrum can also be spread by insect pests, predators, and pollinators, as well as alfalfa pollen. Certain insects, such as the pea aphid (Acyrthosiphon pisum), alfalfa weevil (Hypera postica), migratory grasshopper (Melanoplus sanguinipes), two-striped grasshopper (Melanoplus bivittatus), and alfalfa leafcutting bee (Megachile rotundata) can serve as vectors for effective transmission of the pathogen. Therefore, management of insect vectors should be a part of the strategy for control of this disease. Breeding for disease resistance is thus far the most effective means for control of verticillium wilt of alfalfa. Aspects of breeding efforts for development and commercialization of new alfalfa cultivars in regions of high risk in Canada and approaches to control insect vectors are discussed.

Importance of Pollen and Senescent Petals in the Suppression of Alfalfa Blossom Blight (Sclerotinia sclerotiorum) by Coniothyrium minitans

The effect of pollen and senescent petals on the suppression of alfalfa (Medicago sativa L.) blossom blight (Sclerotinia sclerotiorum) by the mycoparasite Coniothyrium minitans was investigated. When incubated at 20°C for 39 h, germination of conidia of C. minitans and ascospores of S. sclerotiorum was 99.9 and 98.6%, respectively, in the presence of alfalfa pollen (9×104 pollen grains mL−1), whereas spore germination of both organisms was <0.5% in the absence of pollen (in water). In the presence of a commercial pollen product, Swiss™ pollen granules (mainly bee pollen), germination was 99.6% for C. minitans and 98.3% for S. sclerotiorum when the pollen concentration was 1.0% (w/v). When the pollen concentration was reduced to 0.1% (w/v), germination was reduced to 13.0% for C. minitans and 10.8% for S. sclerotiorum. Tests on detached alfalfa florets showed that the colonization of alfalfa florets by S. sclerotiorum was significantly suppressed by C. minitans in the presence of pollen (1.0% Swiss™ pollen granules), especially when C. minitans was inoculated 1-day before S. sclerotiorum. In vivo inoculation tests revealed that the efficacy of C. minitans in the protection of alfalfa pods from the infection by S. sclerotiorum was affected by the time at which C. minitans was applied. When C. minitans was applied on young blossoms of alfalfa at the anthesis stage, pod infection was 96.6% for the treatment of C. minitans+S. sclerotiorum and 99.6% for the treatment of S. sclerotiorum alone. However, when C. minitans was applied on senescent petals of alfalfa at the pod development stage, pod infection was 8.0% for the treatment of C. minitans+S. sclerotiorum compared to 90.8% for the treatment of S. sclerotiorum alone. These results suggest that timing of the application of C. minitans is critical for the mycoparasite to compete with S. sclerotiorum for the source of nutrients from pollen and senescent petals, and for its control of alfalfa blossom blight caused by S. sclerotiorum.

Overwintering of Coniothyrium minkans, a mycoparasite of Sclerotinia sclerotiorum, on the Canadian Prairies

A 3-year field study revealed that Coniothyrium minitans survived Canadian prairie winters and resumed mycoparasitic activities on sclerotia of Sclerotinia sclerotiorum in the following spring when soil temperatures were above freezing. The study concludes that C. minitans has potential as a biocontrol agent of S. sclerotiorum in western Canada because of its ability to overwinter and become active after the winter.

Control, by Brassica seed pomace combined with Pseudomonas boreopolis, of damping-off of watermelon caused by Pythium sp.

A total of 57 isolates of fungi, bacteria, and actinomycetes were screened for their ability to degrade the glucosinolate sinigrin. Pseudomonas boreopolis was the most effective among 26 isolates having the capacity to degrade sinigrin. Treatment with ground Brassica seed pomace and Pseudomonas boreopolis resulted in the release of volatile compounds. Gas-chromatography analyses allowed to identify allyl isothiocyanate and 3-butenyl isothiocyanate as the major volatile compounds released from enzymatic hydrolysis of glucosinolates by Pseudomonas boreopolis. These compounds had strong fungicidal activity against Rhizoctonia solani AG-4, Sclerotium rolfsii, Sclerotinia sclerotiorum, Pythium aphanidermatum, and Phytophthora capsici as revealed by reduced mycelial growth. They were also responsible for reducing the survival of zoospores and oospores of P. aphanidermatum. A large-scale greenhouse trial showed that amendment of growth media (Bas Van Buuren medium No. 4, 7mL; soil, 3 g) with 1% (mass fraction) of the ground Brassica seed pomace and Pseudomonas boreopolis (1 × 108 to 1.8 × 108 cfu/mL) was effective in controlling damping-off of watermelon caused by Pythium sp., resulting in increased production of healthy, marketable seedlings.