Early season infection of onions and barley by Stemphylium vesicarium.

Stemphylium leaf blight is a major foliar disease of onions in Ontario caused by Stemphylium vesicarium. Symptoms often appear at the 3-4 true leaf stage, but inoculum is found early in the growing season. Senesced barley, a wind abatement crop used in Ontario, can be colonized by S. vesicarium. Seeded onions, transplant onions and barley were sampled from field plots in the Holland Marsh, Ontario, Canada, from the cotyledon to the 3-4 true leaf stage and assessed for sporulation. Rotorod and Burkard spore traps began capturing spores in April. Infection was detected in seeded onions beginning at the 2nd true leaf stage on 14 June. Infection of transplant onions generally increased over the sampling dates. Infection was first detected on both senesced and live barley on 9 June and continued to be found at a low frequency. There was a strong correlation between the spores captured by the Rotorod and infection of seeded onions (r=0.79, P=0.02). There was no correlation (r=-0.02, P=0.90) between spores captured by the Rotorod and Burkard spore traps between 18 Apr and 29 Jun. Spores could be useful as an indicator for early season infection and high disease severity later in the season.