Advances in verticillium stripe identification and assessment

One goal of this five-year project was to help farmers and industry predict the impact of verticillium stripe. Other objectives were to facilitate more efficient research into verticillium stripe going forward and to screen genetic material for resistance to the pathogen more effectively. To accomplish all of this, the research team investigated verticillium stripe disease development, evaluated canola genotypes for resistance and examined its interactions with blackleg.

Within this study, researchers measured yield losses in two ways: on a per-plant basis by growing plants in micro-plots inoculated at different densities, and by growing plants in six by 1.5 metre field plots harvested by a small plot combine. They used greenhouse experiments to determine interactions between blackleg and verticillium stripe.

Results

Verticillium stripe infecting canola in the early stages inhibited canola establishment and growth. Later infection led to deterioration of the stem and vascular tissues. Infection at both stages affected yield and yield loss increased as the infections became more severe. Yield loss occurred without obvious symptoms under low moisture.

A 0-6 assessment scale was proposed to evaluate disease severity in canola seedlings, and a 0-4 scale was developed for this evaluation in mature plants. The capacity to assess disease at different growth stages is important for distinguishing between different types of resistance (such as quantitative vs. qualitative) that may be active at different times, and provides a measure of disease progression. Canola breeders could use the scale to evaluate the disease in canola breeding lines.

Researchers also improved methods for inoculating canola with the verticillium stripe-causing fungus, Verticillium longisporum. This is important for screening canola germplasm and more accurately evaluating resistance in canola.

Blackleg or verticillium stripe?

Greenhouse experiments conducted as part of this research found that canola infected with both blackleg and verticillium stripe showed increased blackleg severity, but blackleg didn’t increase the severity of verticillium stripe.

Verticillium stripe symptoms were observed and recorded, including ways to differentiate it from blackleg. This includes:

• Microsclerotia of V. longisporum (the pathogen that leads to verticillium stripe) were much smaller and greyer than the pycnidia produced by L. maculans (the pathogen that leads to blackleg).
• While verticillium stripe and blackleg both cause vascular discolouration, the cross-sections at the base of the stem of plants with blackleg were darker (black) and more discrete than the grey, more diffuse verticillium stripe-infected stems.
• In blackleg-infected plants, the vascular discolouration was restricted to the lower stem, affecting the cortex and epidermis. In contrast, plants infected by verticillium stripe had symptoms extending up the stem, with a hollow, darker centre. When plants were infected by both diseases, longitudinal sections revealed
  a hollow and darker centre together with black discolouration
  of the cortex and epidermis.