High soil pH stimulates the growth of a pathogenic canola fungus

According to Canadian research, alkaline soils especially favour the growth of the fungus, which damages plant tissue by interfering with water and nutrient uptake

by Matteo Cavallito

 

The level of acidity or alkalinity of the soil could play an important role in the growth of a pathogenic fungus, Verticillium longisporum, that is affecting canola crops in western Canada. This is suggested by the results of a research by the University of Alberta. The investigation – the first to assess how these factors affect infection – shows that the microorganism, which causes a disease called Verticillium stripe, grows faster and causes more severe symptoms under neutral to alkaline conditions, i.e. higher pH.

A fungus capable of halving yields

‘Verticillium stripe, caused by Verticillium longisporum, is an emerging disease of canola (Brassica napus) in Canada,’ the study explains. This infection damages plant tissue, interfering with the uptake of water and nutrients. In North America, the pathogen has a rather recent history, having only been discovered in Manitoba canola crops in 2014. Verticillium longisporum was later identified in parts of Alberta, Saskatchewan, British Columbia, Ontario and Quebec.

In these areas, a statement released by the scientists says, the fungus has proven to be a serious problem causing yield losses ranging from 10 to 50 per cent.

With the aim of helping to identify the areas of oilseed rape production in the Prairies most at risk from the disease, the authors conducted an in-depth analysis evaluating different conditions. ‘Fungal growth,’ the research points out, ’was assessed by measuring the colony diameter following 14 and 21 days of incubation on potato dextrose agar at varying pH levels (4.7, 5.5, 6.5, 7.4, or 8.6)’ The results were clear.

Alkaline soils are most at risk

In laboratory experiments researchers evaluated the impact of pH on both the growth of longisporum and its virulence in canola fileds. Results showed that colonies of the fungus were approximately 16% larger in diameter at pH levels of 7.4 and 8.6 than at 5.5. Furthermore, infection symptoms in both seedlings and adult plants became generally more pronounced with increasing soil alkalinity.

‘In a second inoculation experiment, canola seedlings previously inoculated with the fungus were transplanted into potting mix amended to four pH levels (5.6, 6.4, 7.2, and 7.8),’ the research explains.

‘The transplants were cultivated under greenhouse conditions and evaluated for Verticillium stripe severity at plant maturity. Disease severity was greatest at pH 7.8.’ The study, in conclusion, ‘suggests a substantial risk of increased disease severity and yield losses due to Verticillium stripe in regions with neutral to slightly alkaline soils’.

Useful information for countering the pathogen

The survey, in short, provides important information that can help shed light on the phenomenon. The results, the researchers note, help explain, for example, why the disease is more prevalent in Manitoba, which has soils with a more alkaline and neutral pH than Alberta and Saskatchewan. Findings could now help farmers develop more effective strategies for managing the disease.

According to Becky Wang, lead author of the study, knowing more about the pH sensitivity of this pathogen would allow producers and agronomists to develop ‘an integrated crop protection plan for canola’. For example, by avoiding spreading lime on alkaline fields, sanitising equipment, and extending the rotation with longer waiting times before growing the same crop again.