Tomato Blight - Everything You Need To Know
First off, there are a number diseases that affect tomatoes, of which late blight is only one. Others are early blight, septoria leaf spot, white mold, botrytis, etc. Each one is different and requires a different way to manage. The first step is to find out whether you have blight or one of the other diseases.
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
If you have an extended period of wet, rainy weather these techniques will not prevent blight, as the leaves will stay quite wet for days at a time. If you see from the forecast that you are going to have a period of wet weather, especially if it is going to be windy, then a preventative fungicide (like Bordo Mix) may be required. This is of particular importance in July and August when blight pressure is at its worst and the plants are actively growing, producing lots of tender shoots and leaves for the spores to infect.
Here on Prince Edward Island, where we have lots of wet weather and lots of potatoes (which can be a source of blight) we need to put on a fungicide once every 7 -10 days to prevent blight.
If you would prefer to not use a fungicide to control the blight, an option is to grow the tomatoes in a greenhouse or a tunnel made of plastic to prevent the leaves from getting wet during rainy weather.
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, composted (if the pile is hot enough to kill the spores) or tilled into the soil. Thoroughly burying the residue will keep the spores below the soil surface and away from tomatoes.
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.