First Report of Hemp Canker Caused by Sclerotinia sclerotiorum in Alberta, Canada.

Hemp (Cannabis sativa L.) is a herbaceous annual grown mainly for its blast fiber and seed oil. In 1999, Health Canada issued licenses to plant 12,145 ha of hemp in Canada. Of these, 730 ha were in Alberta. During the last week of August, hemp plants (cv. Fasamo) in a central Alberta field showed the following symptoms and signs: wilting foliage turning light brown; dry tan to gray lesions on stems; shredding and breaking of stems at the lesion; presence of white mycelium in the lesion; and black round, irregular, or oblong sclerotia (up to 5 mm diameter and 2 to 11 mm long) present externally at the lesion on the stem and inside the pith cavity. Lesions were found at the crown, near the inflorescence, and along the entire stem length. Disease incidence in a survey of six commercial fields (40 ha) ranged from 1 to 8%. The organism isolated from lesions on potato dextrose agar produced white aerial mycelia and large numbers of sclerotia characteristic of Sclerotinia sclerotiorum. Pathogenicity was confirmed by inoculating 23-day-old greenhouse-grown hemp plants (cv. Fasamo) with autoclaved wheat grains colonized for 14 days with a S. sclerotiorum culture previously isolated from an infected hemp plant. The grains were placed on soilless growing medium near the plant and covered very lightly. One week after inoculation, grayish lesions appeared on the stems, white mycelia appeared on lesions, and plants wilted. The pathogen was reisolated from the lesions. This is the first report of S. sclerotiorum on hemp in Alberta, Canada. The disease known as hemp canker has been reported to cause severe losses under cool wet conditions in the Netherlands (1). Reference: (1) J. M. McPartland. J. Int. Hemp Assoc. 3:19, 1996.

First Report of Powdery Mildew of Greenhouse-grown Tomatoes in Alberta, Canada.

In Alberta, powdery mildew disease of greenhouse-grown tomatoes (Lycopersicon esculentum Mill.) appeared for the first time in 1995 as circular white colonies on leaves, petioles, and stems. Since then it has been found every year and is becoming an economically important disease of tomatoes. This is the first report of the disease from Alberta, Canada. In Canada, powdery mildew on greenhouse tomatoes was previously reported in 1994 from the province of Quebec (1). The pathogen had unbranched conidiophores with average length and width of 62.3 µm and 8.5 µm, respectively. Conidia were clear or hyaline, elliptical to oval in shape, and were borne singly or in short chains. Average length and width of conidia were 36.0 µm and 17.6 µm, respectively. The conidia contained numerous vacuoles but fibrosin bodies were not observed. Germ tubes were straight and formed at the ends or very close to the ends of conidia. Rarely, a conidium produced two germ tubes. Cleistothecia were not found. Six-week-old, greenhouse-grown, healthy tomato cv. Trust plants were inoculated by shaking conidia from powdery mildew-infected plants onto the leaves of the healthy plants. The plants developed powdery mildew symptoms within 9 days after the inoculation. The symptoms on inoculated plants and morphological characteristics of the pathogen were similar to those for naturally infected plants. Based on the characteristics of the asexual stage, the pathogen was identified as an Erysiphe sp. until its identity is confirmed by the characteristics of its sexual stage. An Erysiphe sp. has also been reported to cause powdery mildew of greenhouse-grown tomatoes in Canada (1), the U.S. (3), and Spain (2). Optimal temperature and relative humidity for germination of conidia of the pathogen were 20 to 25°C and >90%, respectively. Myclobutanil, fenarimol, sulfur, triademefon, and triforine showed promise for effective management of this disease. Myclobutanil and sulfur are now registered for control of this disease in Canada. Since cleistothecium has not been found, there is a need to identify the sources of primary inoculum to understand the disease cycle and effective management of the disease. References: (1) R. R. Bélanger and W. R. Jarvis. Plant Dis. 78:640, 1994. (2) L. Olalla and J. A. Torés. Plant Dis. 82:592, 1998. (3) J. F. White, Jr., et al. Plant Dis. 81:227, 1997.

Identification of a Gene Conferring High Levels of Resistance to Verticillium Wilt in Solanum chacoense.

Verticillium wilt (Verticillium albo-atrum) is an important disease affecting potato tuber yield and quality. In North America the major commercial cultivars are susceptible and management strategies for control of the pathogen rely mainly on soil fumigation and crop rotation. In this study 398 genotypes from accessions of Solanum berthaultii, S. chacoense, and S. tarijense were screened for resistance to Verticillium albo-atrum. Resistant genotypes were identified in all but two accessions; however, results indicate that tolerance is more common than resistance. We identified two genotypes in S. chacoense (PI 472819) that had low stem-colonization levels and also did not develop wilt symptoms when inoculated with V. albo-atrum. These genotypes and a susceptible genotype from PI 472810 (S. chacoense) were studied to determine genetic inheritance. Segregation ratios in F1, F2, and backcross populations indicated that resistance in one of the resistant genotypes (18-21R) was controlled by a single dominant gene. Transfer of the Vc gene to tetraploid germ plasm could provide effective and economical control of Verticillium wilt.

Identification of a codominant amplified polymorphic DNA marker linked to the verticillium wilt resistance gene in tomato.

Resistance to verticillium wilt, a vascular disease causing yield losses in many crops, is conferred in tomato by a single dominant allele, Ve. A population segregating for the Ve allele was generated using near-isogenic tomato lines. Analysis of the parental tomato DNA using the polymerase chain reaction and 400 random primers, each 10 deoxyribonucleotides in length, produced 1,880 amplified DNA fragments. Of the four polymorphisms observed between the resistant and susceptible parental genotypes, only one was linked to the Ve gene. No recombination was observed between this DNA marker and the Ve locus, indicating that the linkage is less than 3.5±2.7 cM. The marker detected both the susceptible and resistant alleles, producing amplified DNA fragments of approximately 1,300 and 1,350 bp, respectively. The sequence of the primer, determined from cloned amplified products, was 5' CTCACATGCA 3' instead of the expected 5' CTCACATGCC 3'. The marker will be of value to tomato breeding programs because of the tight linkage, Codominant nature, and analytical procedure utilized.