Penn State professor notes late blight in Md., Pa., N.J., N.Y.

AFP Correspondent

LANDISVILLE, Pa. (Aug. 11, 2015) — During the 2015 Pennsylvania Vegetable Field Day on Aug. 5, Penn State professor of vegetable pathology Beth Gugino emphasized the importance of reporting late blight on both tomato and potato.
Gugino noted that late blight, now confirmed in adjacent states, has been rapidly spreading in New York State.
Her post in Penn State’s vegetable news denotes 10 New York counties plus counties in Maryland, New Jersey and Pennsylvania.
The map on USDA’s website,, shows a July 11 report on tomato in Prince George’s County, Md. That site also contains frequently updated data about late blight. It includes videos which help distinguish the symptoms from other diseases.
Pennsylvania’s first report, Gugino said, was on July 15 on potato. The second was on a potato cull pile. A Somerset county report showed late blight on both tomato and potato.
Gugino also urged submitting samples to the local extension offices so researchers can determine the genotype of the pathogen. Identification of the genotype is highly important to enable effective control of this particularly destructive disease.
Cornell University has been testing samples. So far, US-23 has been identified as the genotype in both tomato and potato this season.
Gugino pointed out that cooler temperatures and longer dew periods, typically in fall, will favor continued development of late blight.
The fuzzy, white spores of late blight are easily spread by the wind.
The pathogen, Phytophthora infestans, requires moisture, which can be provided by rain, dew, fog or irrigation. Even high humidity hastens its spread. High tunnel tomatoes are not immune to its infection.
On tomato plants, late blight begins with small water-soaked areas which enlarge rapidly into purplish-brown oily-looking blotches. The stems develop blackened lesions.
Similar lesions can occur on the aboveground parts of potato plants.
The sporangia on the foliage can be washed down into the soil by rain and irrigation water, then follow the stems and stolons to invade the tubers.
Gugino said grey mold, caused by Botrytis cinerea, can be confused with late blight. Associated with high humidity, grey mold is often observed in high tunnels.
Cool temperatures favor it. The stems commonly become infected through leaf scars or from senescent flowers that contact them. On leaves the irregularly shaped lesions develop wide concentric tan-colored rings. Grayish-brown fuzzy sporulation develops on any infected surface.
Gugino added that botrytis is opportunistic with numerous hosts.
To help in identification, growers can contact their state’s Extension staff. Or, Penn State’s homepage,, has links to images.
Local Extension staffers can recommend the appropriate fungicides for growers to use in their particular area. In addition, they can advise how to rotate treatments to prevent resistance.
Late blight was the cause of the infamous Irish Potato Famine in the 1840s.
In regions of the United States, this fungal disease had occurred sporadically, but became especially problematic over this past decade.