Page 122 - Plant Canada 2024 Proceeding

PLANT CANADA 2024

Salmonberry and thimbleberry are important native plants to Indigenous communities because of their
traditional, cultural and medicinal importance. In 2022, powdery mildew was observed on salmonberry
and thimbleberry plants in research plots of the Agassiz Research and Development Centre, British
Columbia. Although powdery mildew is a common fungal disease on cultivated berry crops, including
strawberry, raspberry, blackberry and blueberry, very little information is available on the diseases
affecting salmonberry and thimbleberry. This research was focused on the identification and
characterization of the pathogen causing powdery mildew on these native berry plants. Symptoms and
signs of diseases including curling of younger leaves and white powdery patches on leaves were
observed on the infected plants. Compound and scanning electron microscopy of the infected leaves
showed the presence of conidia, conidiophores, foot cells and fibrosin bodies. The shape of conidia
ranged from ovo-ellipsoid to citriniform and the conidial size ranged 24.1 to 36.5 µm in length and 12.7–
27.5 µm in width. These morphological features were similar to that of Podosphaera aphanis. For
molecular characterization, DNA was extracted directly from the infected leaf samples taken from each
host. For the amplification and sequencing of the internal transcribed spacer region (ITS), polymerase
chain reaction was performed for the two samples using fungal-specific primers ITS1F and ITS4. Powdery
mildew-specific primers PMITS1 and PMITS2 were also utilized. NCBI BLAST (basic local alignment
search tool) analysis for the sequences from salmonberry and thimbleberry samples showed maximum
similarity (> 99%) to Podosphaera aphanis. For pathogenicity tests, healthy salmonberry and thimbleberry
plants in the greenhouse were inoculated with inoculum from infected leaves of salmonberry and
thimbleberry, respectively. Further, cross-pathogenicity tests were performed by inoculating salmonberry
plants with infected thimbleberry leaves and inoculating thimbleberry plants with infected salmonberry
leaves. One month after inoculation, all inoculated plants started producing white powdery patches on the
upper leaf surface which started spreading to the adjacent leaves. Therefore, cross-pathogenicity to each
host was demonstrated. Further research is required to manage powdery mildew on these native berries.

*[O42] ESTIMATING EARLY INFECTION OF ONIONS BY STEMPHYLIUM VESICARIUM BASED ON
1
2
SPORE TRAPPING AND INFECTION OF BARLEY. Julia Scicluna , Bruce D. Gossen , and Mary Ruth
McDonald . Department of Plant Agriculture, University of Guelph, 50 Stone Rd E, Guelph, ON, Canada,
1 1
2
N1G 2W1; and Saskatoon Research and Development Centre, Agriculture and Agri-Food Canada, 107
Science Place, Saskatoon, SK, Canada, S7N 0X2
Correspondence to: mrmcdona@uoguelph.ca

Stemphylium leaf blight (SLB) caused by Stemphylium vesicarium is an important foliar disease of onion
in Ontario. Symptoms usually appear at the 3-4 true leaf stage, but inoculum is found on spore traps
earlier in the growing season. Senesced barley, which is a wind abatement crop planted with onions, can
be infected by S. vesicarium. Quantifying infection of barley has been used to forecast SLB in Eastern
Canada. Penflufen, a succinate dehydrogenase inhibitor (SDHI) seed treatment, has provided protection
against S. vesicarium infection at early growth stages, but resistance to this fungicide is developing.
Seeded onions, transplanted onions and barley were sampled from field plots in the Holland Marsh,
Ontario from the flag leaf to the 3-4 true leaf stage and assessed for sporulation in humid chambers.
Rotorod and Burkard spore traps were placed in onion fields in April 2023 and 2024. Onion cv. Traverse,
with or without penflufen seed treatment, was inoculated with isolates of S. vesicarium from 2023 at the
st
1 and 3-4 true leaf stages in two controlled environment studies. In field trials, infection was detected in
seeded onions beginning at the 2 true leaf stage. There was a strong positive correlation (r = 0.72, P =
nd
0.03) between the 7-day average of conidia captured by the Rotorod trap and infection of seeded onions.
Infection of barley was detected at a low frequency and was not correlated with infection of seeded or
transplanted onions. There was no effect of penflufen seed treatment on SLB severity in the controlled
environment studies. The Rotorod trap was more effective at capturing conidia than the Burkard trap and
may be useful for predicting the initial infection of onions by S. vesicarium. This could contribute to
disease forecasting models for SLB. Infection of barley is unlikely to accurately forecast the severity of
SLB in Ontario.

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