Manitoba Tomatoes Ravaged By Disease This Year – for Sep. 9, 2010

Nothing beats the taste of a homegrown, vine-ripened tomato, but there are a lot fewer this year due to fungal diseases that have defoliated plants and rotted the fruit.

There’s no controlling it now, but Manitoba gardeners can pick uninfected fruit and let it ripen off the vine. They can also cut out the infected parts if the rot hasn’t spread too far, says Vikram Bisht, a plant pathologist with Manitoba Agriculture, Food and Rural Initiatives (MAFRI) in Carman.

And gardeners can prepare to protect next year’s tomato crop.

First growers need to identify the disease. It will probably be one of three – either early blight (alternaria), septoria leaf spot or late blight. All three can be present.

Late blight is the most destructive. Where tomatoes were decimated, it was likely the cause.

This spring Bisht discovered some young tomato plants being sold at retail outlets in Winnipeg and Brandon were infected with late blight – the same disease that caused the Irish Potato Famine between 1845 and 1852.

The warm, wet conditions, including high humidity, throughout the growing season was “just perfect” for all three diseases, Bisht said. Disease complaints are coming in from tomato growers from across the province, he added.

For small gardeners losing their tomatoes is a major disappointment, but it’s a big economic hit for those with bigger plots.

AManitoba Co-operatorreader from the northern Interlake emailed Aug. 10 to say her tomatoes looked like someone dumped acid on them and the disease was moving into her potatoes.

“Off of 60 tomato plants I saved two gallons” of fruit, she said.

“I usually calculate that the garden produce is worth $2,000 to $3,000 of our grocery bill.”


Manitoba’s commercial potato growers have been routinely spraying their spuds all season at great expense. In most cases their efforts have paid off, Bisht said.

Fungicide spraying will continue until harvest and some potatoes will have to be treated before they go into storage to prevent spoilage.

Late blight will appear anywhere on a tomato plant’s leaves, where as early blight and septoria usually attack the bottom leaves, Bisht said.

The latter cause leaf spots ranging in size from three or four millimetres to 10 mm. Infected fruit can have dark, sunken black spots or cankers.

In contrast, late blight will turn the underside of tomato leaves a purplish-brown colour. Often, white fuzz can be seen on the underside of infected leaves. Infected fruit will have brown spots under the skin.

Most tomato plants have some tolerance to early blight and septoria, but none to late blight.

Tomatoes should be planted in a different part of the garden each year where possible.

If this year’s crop has late blight all the fruit and plants should be removed from the garden, placed in black garbage bags and left in the sun to heat up and destroy the fungal disease. There’s a risk of infecting next year’s crop if infected plants are worked into the garden or even composted, Bisht said.

Such measures aren’t as necessary in the case of early blight and septoria, which aren’t as destructive.

Bisht says gardeners should be vigilant next year. Any early signs of infection should be pruned and removed, he said.

Growers can also protect their tomatoes by applying a fungicide containing either mancozeb or copper, available at retail garden outlets.

Such fungicides help protect plants from becoming diseased, but won’t cure an already-infected plant. That’s why it’s important to spray at the first sign of infection or when nearby gardens are infected, Bisht said.

While fungicides are relatively safe be sure to follow label directions, wear protective clothing and prevent children and pets from being exposed.

It’s likely where tomatoes are infected with late blight; potatoes in the same garden will be too, Bisht said. Infected tubers will spoil in storage. Those that don’t shouldn’t be planted for seed next year to avoid reintroducing the disease.

Infected potato plants should also be destroyed.

It’s best to buy new, certified, disease-free potato seed every year, Bisht said. [email protected]

About the author


Allan Dawson

Allan Dawson is a reporter with the Manitoba Co-operator based near Miami, Man. Covering agriculture since 1980, Dawson has spent most of his career with the Co-operator except for several years with Farmers’ Independent Weekly and before that a Morden-Winkler area radio station.



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