WHAT IS LATE BLIGHT?
Late blight is a disease caused by the fungus Phytophthora infestans. This fungus effects potatoes as well as tomatoes, and was the cause of the Irish Potato Famine in the 1840s. It can spread like wildfire, as the pathogen can produce huge numbers of infectious spores that will travel on the wind. There's no cure for late blight - infected plants must be destroyed immediately, before more plants are effected; spore germination to symptoms takes about 7 days, so immediate action is of the utmost importance. The good news is that with proper planting, husbandry and preparation, it's preventable.
BEATING BLIGHT
There are several things you can do to help prevent blight. First, look for tomato varieties that are blight resistant. While no tomato is completely immune, breeders are working on varieties that are less susceptible to the fungus - Mountain Magic and Plum Regal are two (and more info on resistant varieties here). Planting early-maturing varieties is also a good strategy; that way, you may be able to get at least a modest harvest before late blight "season" rolls around.
The way you care for your plants is important, too. Cool, wet conditions are perfect for the development of blight, so the following guidelines will create a "blight-unfriendly" environment:
- Stake or trellis your tomato plants to maximize light and airflow around the plants.
- Be sure to allow enough space between plants. Dwarf varieties can be spaced about 12" apart, determinate/bush-type plants can be spaced about 24" apart, but sprawling, indeterminate varieties may need 36" - 48".
- Water with drip irrigation, soaker hoses or by watering the soil - NOT the plant! Water early in the day so the plant is dry when the sun goes down and temperatures drop.
- If you grow potatoes, but sure you rotate your planting and don't plant tomatoes in the same spot you grew potatoes the year before (or potatoes where your tomatoes were the previous year).
- If you missed a few potatoes during last fall's harvest and some volunteer plants show up this spring, destroy them. While it's believed that the fungus can't over-winter in the soil, it can over-winter in potatoes, so best to be safe and destroy them.
- It's also believed that the spores can't survive in dead plant material, but until the plant dies, the spores are alive and kicking - so remove and destroy diseased plants immediately. Bag them in black plastic or cover them with black plastic or tarp until they're good and dead, then throw them in the trash - do NOT compost diseased plants!
- Preventive spraying with an organic product like Actinovate (which contains Streptomyces lydicus, a beneficial bacteria) may help prevent blight. Copper fungicide should be applied when late blight is present to keep it from spreading to healthy plants. Bonide (which we carry) has great info on copper fungicides and their use on their web site.
It's also important to know late blight when you see it. There are some conditions that appear similar - early blight and septoria leaf blight are two, and drought-stresses plants can show similar symptoms as well. And please remember, if you have late blight in your garden, let your neighbors know as soon as possible - since the spores are windborne, they can travel to neighboring gardens; putting your neighbors on alert will allow them to start preventive treatment before their crops can be damaged.
Learn more about late blight and how to recognize it at USABlight.org and http://www.longislandhort.cornell.edu/vegpath/photos/lateblight_tomato.htm.
