Features Production Vegetables
Biofumigants and fumigants being assessed for tomato crops

Root rot in tomatoes can be a blight on a grower’s bottom line, leading to early defoliation and vine decline.

Root rot in tomatoes can be a blight on a grower’s bottom line, leading to early defoliation and vine decline. In 2009 and 2010, a number of tomato fields in Essex and Kent County reported corky root rot and vine decline symptoms. Along with common root rot fungi such as Fusarium spp. and Colletotichum coccodes, which cause black dot, three other fungi – Pyrenochaeta lycopersici, Pyrenochaeta terrestris, and Rhizopycnis vagum – were identified from these fields. The last three fungi are believed to be associated specifically with corky root rot, says Cheryl Trueman, professor of vegetable pathology and entomology, Ridgetown Campus, University of Guelph.

These pathogens produce microsclerotia, which can survive in the soil for a long time. To determine if fumigation and biofumigation could help reduce the effects of these pathogens, field trials were conducted in 2010 by Trueman and fellow researchers Anne Verhallen, a soil management specialist, and Janice LeBoeuf, a vegetable specialist, both with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) and based at Ridgetown.

In 2010, the fumigant Vapam was used to treat soil infested with the vine decline pathogens at the highest recommended rate. In the greenhouse, tomatoes were placed in pots of treated soil and root symptoms were later assessed, explained Trueman. In another trial, soil at a commercial site exhibiting vine decline symptoms was treated with Vapam at both the low and high recommended rates and compared to a non-fumigated treatment. Finally, Trueman collaborated with Dr. Ken Conn at the Agriculture and Agri-Food Canada (AAFC) Southern Crop Protection and Food Research Station in London, Ont. Here, soil from the commercial field site was treated with the low and high rates of Vapam, and placed in outdoor microplots. The microplots allowed for uniform mixing of the soil while still allowing research to be conducted in an outdoor environment.

To assess the effectiveness of the treatments, researchers counted the number of banded root lesions per gram of dried root. At six weeks after planting, they found fewer lesions on plants grown in soil treated with Vapam in the greenhouse, but no significant difference between plants treated at either the high or low rate in the microplots or the commercial field trial. Tomato yield in plants grown in soil treated with the high rate of Vapam was higher than the nontreated in the microplot trial, but not the field trial.

“Vapam produced inconsistent results,” Trueman said, adding, “we need to repeat the trials again in 2011.”
In 2010, Vapam was also compared with a biofumigant crop and untreated check plots. Trials were conducted at five different sites.

LeBoeuf defines a biofumigant as a variety of brassica crop that contains glucosinolates that, when crushed and mixed with plant enzymes, creates isothiocyanate. This compound is similar to the active ingredient found in Vapam.

“It’s essentially nature’s Vapam,” she said.

For the trial, oilseed radish was the biofumigant crop.

Given Ontario’s short growing season, spring seeding of brassica crops is not practical. The plants do not have enough time to grow to the point where they can be an effective biofumigant, LeBoeuf said. Early fall seeding is preferred.

In the spring of 2010, the crop was finely chopped, then well incorporated into the soil.

“Chopping is very important,” said LeBoeuf. If you don’t chop, nematode populations can actually increase.”

Ideally, chopping and soil incorporation should occur two weeks before planting. This gives the isothiocyanates enough time to work against the pathogens, and slowly reduce in strength so no harm is done to the tender roots of the tomato transplants.

To assess the effectiveness of the treatments, the research team monitored crop growth, vine decline and yield. They also looked at banded root lesions, nematode populations and disease pathogens in the soil.

Given the sheer size and scope of the 2010 trial, the researchers have not arrived at any conclusions just yet.

“We have tons of data left to analyze, and it will be published on our website as soon as it becomes available,” said LeBoeuf.