Irish Potato Famine Disease affecting Gardens in our area

July 9, 12:00 PMPhiladelphia Home Gardening ExaminerMelissa Miles

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  Tomato with blight (photo credit:  Cornell University)

"Late blight" (Phytophthora infestans), the same disease that caused the Irish Potato Famine of the 19th century, is spreading through the north eastern U.S. and affecting tomato and potato plants in gardens and farms throughout the area. 

Though the disease is not harmful to humans, it has been deemed "worse than Bubonic plague for plants" by Cornell University's Meg McGrath, professor of plant pathology.  Late blight infections occur sporadically, but this year the disease is hitting the area earlier, and with more intensity than ever before. A  late blight infection spells almost certain death for tomato plants in home gardens. 

The cool weather and frequent rain the eastern U.S. has experienced in recent months have created ideal conditions for blight. Because late blight is highly contagious, the out break could have been caused by garden plant retailers selling infected tomato plants - particularly the 'big-box' stores.  For this reason several stores, including Wal-Mart, Home Depot and Lowes have removed tomato plants from shelves in their eastern U.S. stores.  If you purchased plants (rather than growing them from seed) for your garden this year, you may have purchased an infected plant. 

Here's what you can do:

Recognizing Late Blight - plants should be inspected daily

Brown spots on the stems of tomato plants is generally the first sign of a blight infection.  Later, nickel sized splotches of brown or olive green may appear on leaves, followed by fuzzy white growths on the underside of leaves.  Infected plants will have tomatoes with firm, brown spots. (See article slide show).

Diagnostic Services:
For home gardeners interested in getting diagnostic testing for late blight on tomato (or other diseases on vegetables in their garden)

In Pennsylvania:
Obtain a diagnosis form and envelop from your County Cooperative Extension office. Fill in the form and mail the sample on a day when it will reach University Park promptly without sitting in a post office over a weekend. Or, send the sample via overnight delivery service to:

Plant Disease Clinic
The Pennsylvania State University
Dept. of Plant Pathology
218 Buckhout Lab.
University Park, PA 16802-4507

General diagnoses are done at no charge. (If forwarded to a commercial lab for virus or other specialized testing, the cost of that testing is passed on to the client.)

In New Jersey:
Samples should be sent along with a submission form available through all New Jersey County Extension Offices or by FAX (908-932-1270) Mail the sample to:

Plant Diagnostic Laboratory
Rutgers Cooperative Extension
PO Box 550
Milltown, NJ 08850-0550

What to do if you suspect late blight infection?

if you suspect late Blight in your vegetable garden you must quickly decide whether you wish to spray your plants with a fungicide (that is specifically designed for blight AND tomato or potato crops - it should contain chlorothalonil), or if you wish to destroy the infected plants entirely. 

If you are attempting to control blight with fungicide, remove infected leaves, and place them in a resealable plastic bag for disposal prior to applying the chemical. Use the fungicide as directed on the label.

If you choose to remove the plant to stop the spread of the disease, place the infected plant in a resealable bag, and dispose of with your household trash

Since spores carrying the disease can become wind-born, thus spreading the disease to neighboring plants, try to disturb the plant as little as possible.  Composting of infected leaves or plants is NOT recommended!

The Department of Agriculture is investigating the cause of the outbreak, but evidence seems to indicate that this may be another instance in which buying local - getting your garden transplants from smaller, local retailers, rather than the large, chain stores (who use a central product supplier, thousands of miles away) - could have prevented this, potentially devastating, out break of disease.