Fruit & Vegetable Magazine

Features Production Research
Research investigates management of black rot on butternut squash

management of black rot on butternut squash


March 13, 2008
By Marg Land


Topics

It all started a few years ago
with a strange looking butternut squash. Ontario Ministry of
Agriculture, Food and Rural Affairs (OMAFRA) vegetable specialist
Elaine Roddy observed the squash’s strange skin symptoms, similar to
alligator skin, while in the field.

It all started a few years ago with a strange looking butternut squash. Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) vegetable specialist Elaine Roddy observed the squash’s strange skin symptoms, similar to alligator skin, while in the field. Later, she described her finding to Dr. Mary Ruth McDonald, an associate professor and researcher with the University of Guelph’s Department of Plant Science. Roddy described the skin blemish as being very similar to the symptoms for black rot, a fungal disease which can be a problem on cucurbits grown in greenhouses.

That’s exactly what the disease turned out to be.

Advertisment

“It’s not as commonly seen in the field,” explains Dr. McDonald.

maryruth
Dr. Mary Ruth McDonald

Black rot is caused by the fungal agent Didymella bryoniae, which is also known as gummy stem blight when the disease is found on the plant. It’s called black rot when the disease is observed on the fruit.

The fungus can be spread through infected seed. It usually overwinters on crop debris and can survive for 1.5 to two years in the field. Spores are produced in the spring and ooze out of the plant debris where they can then be splashed up onto surrounding plants through rain. The spores can also be transferred via the wind. For infection to occur, the fungus needs one to 10 hours of leaf wetness, with the optimum conditions occurring between 18 and 25˚C.

Any part of the plant can be infected.

According to Dr. McDonald, the best way to avoid the disease is:
•    To only plant disease free seed from
certified seed sources
•    Use a crop rotation of two to three years
•    Deep plow crop debris right after harvest
•    Control powdery mildew and cucumber beetles within the crop as disease and insect damage can provide an alternative route for black rot to enter the plant or fruit
•    Begin a fungicide spray program as soon as powdery mildew is observed in the crop since powdery mildew and development of black rot are linked

cahrt2

During the 2006 growing season, Dr. McDonald examined several fungicide spray programs – including Pristine, Switch and Quadris – for effectiveness in controlling gummy stem blight and black rot on butternut squash. Currently, only Lance and Mancozeb are registered for control of gummy stem blight in Canada.

Plots were established at the University of Guelph’s Muck Crops Research Station in the Holland Marsh, located north of Toronto. The butternut squash plants (Avalon variety) were innoculated with an isolate from fungus on August 3 and August 25.

While the results showed that Pristine has potential as effective fungicide for controlling black rot on squash, variable levels of disease and fruit yield in the plots resulted in no statistically significant differences among the treatments.

“We’re still not really sure why it was so variable,” says Dr. McDonald. “We hope to find a solution for this in 2007. Meanwhile, (growers) can expect very variable development through the field.”

Dr. McDonald also examined two spray programs: one with about a 10-day interval (five sprays total – July 31, August 11, 18, 28, September 7); the other with about a 20-day interval (three sprays in total – July 31, August 18, September 7). Results showed there was no advantage to following a five-spray program over a three-spray one.

Dr. McDonald hopes to continue research in 2007, and plans to closely examine the wide variability of infection rates in the field observed during the 2006 season. She is also interested in the possibility of forecasting the disease.

“There is no disease development if there is no leaf wetness,” explains Dr. McDonald, adding there may be potential to develop a forecasting model for disease severity, and the proper timing of sprays, based on leaf wetness.

Other researchers involved in this project include Mike Celetti, plant pathologist, OMAFRA; Dr. Greg Boland, professor, Department of Environmental Biology, University of Guelph, and Dr. Bruce Gossen, research scientist, AAFC, Saskatoon.