The pathogen Phytophthora infestans, commonly called Late Blight or Irish Potato Blight, has been responsible for the demise of potato, tomato, and other solanaceous crops worldwide for centuries. It was the vicious culprit of the European, Irish, and Highland Potato Famines in the 1840’s and, more recently, the widespread epidemic in the Northeastern U.S. in 2009. This pathogen strikes when weather conditions are moist (high humidity, fog, dew, rain, etc.) with nighttime temperatures of 50 to 60 degrees F and daytime temperatures of 60 to 70 degrees F for 4-5 continual days. Late Blight is essentially a water mold that, under these conditions, can spread rapidly, defoliating your entire crop within 3 days to 3 weeks. The first symptoms are irregular grayish spots on leaves that have a water soaked appearance. Eventually, all plant tissue will become necrotic and fruit and tubers will develop legions and rot. Because it is very difficult to manage organically, let’s focus mainly on prevention.
Your first line of defense should be to try to find tomato and potato varieties that are resistant to Late Blight. Currently there are not a whole lot of choices for organic tomato seed and seed potatoes, but there are many plant breeders that are currently working on new resistant varieties and more should be available soon. In my experience, as well as many of our customers’, Matt’s Wild Cherry Tomato is a High Mowing Organic Seeds variety that shows great resistance. If you insist on planting your favorite standby varieties, there are other things you can do to help you prevent this disease in your garden. First, choose seed potatoes that come from certified seed potato producers. Certified seed potatoes are monitored closely by professionals that can spot early signs of disease making you less likely purchase diseased seed. You do not have to worry about tomato seed, as tomato seed cannot transmit Late Blight.
Late Blight can only survive on living plant tissue; therefore you should begin by planting in an area where there are no current volunteer hosts, such as tomato, potato, eggplant, or peppers. If possible, avoid planting in the same area where these crops were planted last season, as the spores can over-winter in areas with mild, frost-free winters. And, at the end of the season, be sure to remove and destroy any diseased plant matter, fruit or tubers. However, if you live in an area with very cold winters, you can spread the plant matter on the soil surface and the spores will not survive.
Knowing that P. infestans thrives in moist conditions, you will want to do all you can to be sure your plants have adequate airflow and can dry out from the morning dew or after rainfall. Choose an area of your garden or a field with good air circulation, well-drained soil, and areas that are not shaded by trees or structures. You can also orient your planting rows with the prevailing winds to increase airflow, making sure to allow adequate space between plants. Staking and pruning your tomatoes will also increase airflow, but be sure not to do this when the plants are wet. If you are irrigating, drip irrigation is better than overhead irrigation because it keeps that plant dry. If you have to use overhead irrigation, choose your time of day carefully because you want to be sure the plants have a chance to dry out before nightfall. And hilling your potatoes high will help to protect the tubers below from being exposed to any spores that might leach though the soil with rain or irrigation water. Another thing to consider is crop nutrition. An oversupply of nitrogen will encourage a large, lush canopy that produces less fruit and will also take much longer to dry out. And lastly, be sure to manage your weeds, as weeds impede airflow as well.
Once you spot the symptoms of this disease in your garden, it is likely too late to reverse the effects, but there are organic controls that you can use preventatively. Spraying your plants with compost tea or other foliar feeds, especially ones formulated with kelp, can help to suppress the blight. By adding beneficial bacteria, you are inoculating the leaf surfaces with microorganisms that act rivals to the invading pathogens and make it difficult for them to get started. Similarly, Serenade biofungicide is a wettable powder that inhibits the attachment of the pathogen, stops it from growing, and helps develop resistance in the plant. Other organic sprays that act as preventatives are copper spays such as Champion WP and Storox, a hydrogen-dioxide based pesticide (which has shown variable results as a curative as well). And remember, all sprays require complete and regular coverage of all plant surfaces to be effective.
Now that you are in the know, you can enjoy helping your garden grow!
For further information on Late Blight, check out eOrganic eXtension’s Late Blight and its Management in Organic Gardens webinar.
All pictures were taken from Cornell University’s website. For these pictures, as well as other pictures of Late Blight, please visit their Research and Extension website.