Good Gardening Practices Can Keep Blights At Bay

By Nancy K. Crevier

Last summer’s infestation of late blight, a destructive, wind-borne pathogen that spreads rapidly in tomato and potato crops, was unusually devastating to gardeners in Connecticut. With tomato seedlings already sprouting on windowsills all across the state, concerns that this summer will prove equally discouraging are cropping up. “We’ve been flooded with phone calls from people worried that the disease has over-wintered in the soil,” said Joan Allen, assistant cooperative extension educator at UConn’s Home and Garden Education Center in Storrs.

The plant pathologist has good news, though. “In our area, late blight only survives in living plant tissue. It is not likely to survive in the soil over the winter,” she said. For late blight to take hold of a region, said Ms Allen, there are three things that must occur. First, there must be a susceptible host — in this area, usually tomatoes, a favorite garden plant, or potatoes. Secondly, a virulent pathogen must be introduced. Last year it is believed that late blight arrived on plants imported from Southern states to big box stores in Connecticut.

“Symptoms of late blight, early on, can be so minor that they can be overlooked. It’s important to inspect plants for insects or disease when purchasing them, but that’s the best you can do,” Ms Allen said. “Sometimes it is just not possible to identify it in the early stages,” she said.

The third factor for late blight infestation is a favorable environment in which the pathogen can thrive. “Blight needs a film of water on the leaves in order to infect the plant, and last year we had very wet, cool weather. Some people say that it was the ‘perfect storm’ for late blight infestation,” Ms Allen said.

Late blight, or Phytophthora infestans, will not survive Connecticut’s freezing ground temperatures, but any infected potato tubers remaining in the ground from last season can harbor the pathogen. “You have to destroy any of those tubers,” Ms Allen warned. The best way is to dig them up, bag them in plastic bags securely closed, and bring them to the dump. Burying the tubers a minimum of two feet deep can also destroy the pathogen. The pathogen is unable to travel to the surface from that depth and will not survive. “If you find that you’ve missed some of the tubers and they germinate, pull them up and destroy them,” said Ms Allen.

While gardeners are unable to control Mother Nature, good gardening practices can help discourage the onset of late blight or other diseases. “You are less likely to get disease if you start with healthy transplants, for one thing,” Ms Allen suggested. “What really helps is keeping leaves dry.” Growing tomato plants under a top cover and watering only from the bottom is a good practice, she said.

Crop rotation can also discourage blights — some of which are able to over-winter in Connecticut soil — from infecting plants in the garden. Ideally, a tomato crop would be rotated out of an area every three or four years, said Ms Allen. Small gardens can be divided into quadrants, she said, and tomato or potato crops rotated between those four areas.

There are a few tomato varieties that have shown some resistance to late blight or other blights, mostly based on anecdotal stories and not full research, Ms Allen said. They include Stupice, Juliet, Matt’s Wild Cherry, Mountain Magic, Plum Regal, and Legend. “Some research studies have actually shown that Legend is resistant,” Ms Allen said.

When only one or two of the favorable conditions exist, late blight does not become a problem. “I’m sure there will be more of an awareness this year,” said Ms Allen, “but hopefully we won’t have the influx of infestation that we did last year.”

For more information contact the UConn Home and Garden Education Center at 877-486-6271, 860-486-6740, or email ladybug@uconn.edu. Soil testing kits are available at the Fairfield County Extension Service, 67 Stony Hill Road (Route 6), Bethel, 203-797-4176.