Are Stemphylium spp. seed borne pathogens of pea (Pisum sativum L.)?

The effects of Stemphylium spp. on pea seeds and plants were studied in four controlled environment and laboratory experiments. Laboratory seed health testing of commercial pea seed lots established that Stemphylium spp. can be seed borne in peas, with infection levels varying from 0 to 46% depending on production season and cultivar. Stemphylium spp. were isolated from naturally infected marrowfat pea seeds, grown on artificial culture media, and their characteristics described. Molecular tests performed by the Plant Pathology Laboratory, USDA/ARS (USA), confirmed the five main species isolated were: S. loti Graham, S. vesicarium (Wallr) Simmons, S. herbarum Simmons, S. astragali Yoshii and S. sarciniforme (Cavara) Wiltshire. Stemphylium spp. did not kill seeds or affect laboratory seed germination. The species of Stemphylium isolated from seeds are likely to vary in their pathogenicity and ability to reduce viability. Of the isolates, an isolate of Stemphylium herbarum produced the most conidia. This isolate was tested in a third experiment in order to determine its pathogenicity on pea plants. Plants at three different stages of development were inoculated with sterile distilled water, sterile distilled water plus Tween, S. herbarum conidial suspension and S. herbarum conidial suspension plus Tween. Under the conditions (mean temperature of 19°C and l00% relative humidity) S. herbarum caused lesions on pea leaves, especially in the early stages of plant development (seedlings at the 3- 5 node stage). Symptoms observed were brown, irregular to oval shaped lesions, approximately 6mm in diameter. On a scale of 0-5 seedlings inoculated with conidial suspension of the fungus had a mean disease score of 2 while untreated plants had a disease score of 0. The mean leaf diseased tissue area was also higher for seedlings inoculated with S. herbarum conidial suspension than for untreated seedlings (29 mm² cf. 0 mm², respectively). Scanning electron microscopy revealed that S. herbarum penetrated the leaf tissue via the epidermis as well as the stomata. Infection was more successful on seedlings than adult plants. Stemphylium herbarum was re-isolated from infected tissue, thus fulfilling Koch's Postulates. Movement of the fungus from infected leaf tissue to seeds was not demonstrated. Infected seeds may be the first source of inoculum of Stemphylium spp., ensuring the perpetuation of the fungi and their spread to new areas. Experiment four tested the effectiveness of registered seed treatments for peas (Aliette super, Apron XL and Wakil XL) and a hot water soak treatment (50 ± 0.5°C for 30 minutes) for control of Stemphylium spp. Chemicals had limited success in controlling Stemphylium spp. infection. Hot water treatment was the most effective method, eliminating l00% of Stemphylium spp. from the seed lots tested, but it was harmful to the seeds, reducing germination by approximately 34%.