late tomato blight

(Question)

I have been producing tomatoes on a modest scale for 30 years in South Bruce (Teeswater). This has never been difficult until two years ago when we were hit with a an airborne late tomato blight. We were affected again last year. As the fruit is ripening it goes rotten.I have planted 35 plants this year and do not want a repeat of this.
Do you have a solution?
Any help will be gratefully received.
Many thanks.

(Answer)

The fungus Phytophthora infestans causes late blight disease. Outbreaks spread quickly under favorable conditions and  once a plant is infected, it must be destroyed.

The first sign of symptoms on plants is often a brown/black lesion on the stem or petiole. Leaves develop large brown/black blotches, often starting at leaf margins. In humid weather and in early mornings, a fuzzy mould can often be seen on the underside of the brown/black blotches or on the stem lesions. This fuzzy growth contains spores. These spores can travel up to 20km in wind or wind-blown rain and can spread from plant to plant due to splashing water.

In the last few years, late blight has become a major threat to both home gardeners and commercial growers. Unfortunately, there’s no single control for this disease.

If you do have late blight there are a few things that you can do to reduce the amount of blight you have. Unfortunately, once you see symptoms on your plants there is little you can do that year. If you catch it very early you can start by picking off the leaves to keep it from getting worse but this only slows it. A preventative strategy is required.

Blight is a disease spread mostly by wind borne spores. Some can overwinter in infected plant debris in your garden, so cleaning up in the fall and rotation are a good idea, but will not prevent it totally as they can blow in from several miles away.

The spores need the leaves to be wet for several hours so that they can germinate and infect the plant. Anything that you can do to prevent the leaves from being wet will help, for instance:  plant in an area where the plants will get good air circulation, – water in the morning so the leaves have a chance to dry off during the day – prune the plants to keep them from becoming too bushy – grow them in a sheltered location (like against the house) where they get less dew and rain on them – keep the plants up off the ground

Proper sanitation measures can keep spores from infecting the next crop. At the end of the growing season all tomato refuse should be removed and discarded.

Crop rotation is another means to help reduce disease in tomato plantings. Each year plant tomatoes in a new location away from areas where tomatoes, eggplant, potatoes or peppers have grown in the past. These vegetables all have similar disease problems. A minimum rotation of three years is considered essential to help reduce populations of soil-borne fungi.

A second line of defense against leaf spot diseases is to alter the microclimate surrounding tomato plants. Fungi thrive in moist, humid conditions, in particularly on leaves that remain wet for long periods of time. Tomatoes should be grown in full sun with good air circulation to dry the leaves. Staking or caging tomatoes brings the plants up off the soil and allows more rapid drying of the plant.

You might consider planting resistant varieties. Though no tomato varieties are completely immune to late blight, plant breeders are now developing varieties that are resistant to infection by the late blight fungus. Here is a link to late blight resistant varieties.

Here is an excellent article put out by North Carolina State University on Light Blight in Tomato plants.