Late blight comes early to Wisconsin potatoes, tomatoes

This is a publication from AGRI-VIEW, the Wisconsin’s leading agricultural newspaper

By Jane Fyksen Crops Editor of Agri-View

Photo by Jane FyksenLate blight – a potentially devastating disease to potato and tomato crops – is once again in Wisconsin. Farmers and backyard gardeners alike need to be scouting their crops, says Ken Schroeder, Portage County ag agent, of what’s often called a “community disease,” as it easily spreads neighbor to neighbor by spores, via wind, rain, machinery, workers and wildlife.

This disease thrives under wet conditions, which many parts of Wisconsin has more than their fair share of this season.

The UW-Madison Plant Disease Diagnostic Clinic has, as of Agri-View’s Monday press deadline, confirmed the presence of late blight (caused by Phytophthora infestans) on commercial potatoes in Adams and Juneau counties and on tomatoes in Sauk County. In all cases, the late blight has been genotyped as US-23.

Late blight is a fungal-like disease that very serious when it infects potatoes and tomatoes, as well as occasionally egg plant and nightshades. It was the cause of the Irish potato famine of the 1850s, notes Schroeder.

“Left unmanaged, a small outbreak can lead to an epidemic devastating gardens and commercial vegetable fields. Farmers who grow potatoes and tomatoes are at serious risk of losing their entire income for the season,” warns Schroeder. “This disease has the potential to completely defoliate fields within three weeks of the first visible infections.”

Leaf symptoms, on both potatoes and tomatoes, appear as pale green, water-soaked spots that often begin at leaf edges or tips, where water from rain and dew accumulates. Lesions can be circular or irregular, and bordered by pale yellow to green, blending into healthy tissue, Schroeder describes. Lesions enlarge quickly (expanding one-quarter to one-half inch per day) and turning brown to black and becoming oily-looking with time.

Tomato fruit symptoms begin small, but quickly develop into golden to chocolate-brown firm lesions or spots that can appear sunken with distinct rings within them; the pathogen also can sporulate on tomato fruit giving the appearance of white, fuzzy growth. The time from first infection to lesion development and sporulation can be as fast as seven days, depending upon the weather.

With relative humidity in excess of 90 percent, leaf lesions are often surrounded by cottony white mold on the lower leaf surface. This white, cottony growth distinguishes late blight from several of the foliar diseases of potatoes and tomatoes, notes Schroeder.

Late blight comes early to Wisconsin potatoes, tomatoes

Photo by Jane Fyksen

Infected stems and petioles turn brown to black and also may be covered with white masses of “sporangia,” he continues, noting that stem lesions frequently appear first at the junction between the stem and leaf, or at the cluster of leaves at the top of the stem. Entire vines may be killed very rapidly, and Schroeder says that a “characteristic odor similar to that produced by green tissue after a severe frost can be detected.”

Commercial growers are hopefully already applying preventative treatments, and checking potatoes and tomatoes closely for late blight symptoms at least twice a week. Farm families growing potatoes and tomatoes in their home gardeners also should be scouting and if they suspect late blight contact their local Extension office and have a sample sent to the UW’s Plant Disease Diagnostic Clinic for confirmation. Preventative treatments for homeowners are limited to protective fungicides containing Chlorothalonil or copper.

“Once a plant is infected, these treatments won’t kill the fungus. They are preventative, not curative,” says this ag agent.

He adds that infected garden plants should be destroyed by putting them in plastic bags for disposal. They should not be composted.

Commercial growers should consult UW-Extension publication A3422, Commercial Vegetable Production in Wisconsin, for an extensive list of available treatments. It’s online at Commercial Vegetables Production in Wisconsin

Schroeder also warns of late blight “look-alikes.” One of them is early blight, which appears as brown-to-black lesions with concentric rings on the leaves. Typically early blight lesions are produced on older lower leaves; the disease progresses upward. Significant yellowing may accompany the lesions. Moderate temperatures (75 to 85 degrees), high humidity and prolonged leaf wetness are conducive to early blight development, and alternating periods of wet and dry weather tend to increase progression of this disease. He directs growers to a UW fact sheet on early blight online

Yet another disease that can be confused with late blight is botrytis/gray mold, that appears late in the season on foliage of many plants. It creates a grayish-green, wedge-shaped, spreading lesion with concentric rings on the leaves that, like late blight, begin on the margins or tips of the leaves. With severe gray mold infection, leaves are “blighted” and a soft gray rot attacks stems and exhibits a fuzzy gray fungal growth, says Schroeder. When vines are disturbed, spores billow from them like a cloud of dust. Cool temperatures and high humidity promote gray mold, which he says is often found in fields where a lot of fertilizer is used.

“Typically, gray mold is not of economic importance in Wisconsin,” he adds and directs growers to a fact sheet on gray mold online 

Schroeder says a very common leaf disease of tomato, Septoria leaf blight, is not necessarily an easily mistaken look-alike. Symptoms start on foliage closest to the ground and move up the plants. Leaf spots are small and circular with dark borders and gray or tan centers; eventually, multiple spots on a single leaf will merge. Warm, wet and humid weather increases the severity of this disease, which can progress to the point where all foliage is killed and falls from the plant. See a Septoria leaf spot fact sheet.

Amanda Gevens, UW-Madison vegetable plant pathologist, admits that the Wisconsin’s late blight infection is “challenging to know with certainty where the initial inoculum may have come from,” though potato field symptoms do not suggest a seed-borne source, as no stem lesions were observed and the incidence of lesions across the field suggests “deposition by a spore shower.” No potato volunteers have been noted either to suggest that as the potential source. And cull piles do not appear to be the source either.

She is recommending a shortened 5 to 7-day spray interval for preventative fungicide applications, and in order to “help better understand the epidemic at hand,” she urges growers to submit samples to her laboratory for genotyping by sending infect leaves in a slightly inflated zipper-locked plastic bag (no paper towel) to: Amanda Gevens, 1630 Linden Dr., Room 689, Plant Pathology Dept., University of Wisconsin, Madison 53706.

She says US-23 can be managed with mefenoxam fungicides, though there is been some variability in sensitivity among US-23 isolates collected from Wisconsin in previous years. Gevens says late blight has been identified on tomatoes and potatoes in Wisconsin in each of the last four years. She details that the aggressive US-23 produces roughly twice as many spores per lesion as other late blight genotypes and “has great potential to rapidly reproduce and spread.”

Gevens says the disease-forecasting tool (Blitecast) indicates risk times for late blight activity and can aid in identifying critical times for preventative fungicide applications. Access is via She warns home gardeners that fungicide must be present on foliage to have a protective, disease-limiting effect. And because new growth is not protected, and fungicides can wash off, repeat sprays are necessary. Research shows the best organic-approved fungicide for potato late blight control is copper.

In the past two years, Gevens says the UW has been investigating efficacy of non-copper organic fungicides and have demonstrated good control of tomato late blight with EF-400 under laboratory conditions.

Michigan State University has conducted field trials with EF-400 plus ExCit (now BacStop) on potatoes and corroborated the UW’s laboratory efficacy results. Good field control of potato late blight was demonstrated with weekly applications of EF-400 plus ExCit for eight weeks.

Additional information on these organic products can be found at

Gevens says the UW also tested several organic fungicides (and made a few conventional comparisons). Zonix (a rhamnolipid from Jeneil Biosurfactant) and EF400 (US Agritech) performed well when applied before inoculation (prior to disease onset).

“Fungicides have the best chance of effectively managing disease when applied before disease starts; this is true for all fungicides, conventional and organic,” she emphasizes. Gevens adds that while Oxidate did not perform well, “keep in mind that it is a contact anti-sporulant and will kill spores on contact, but will not provide lasting control as a protectant. It has a place to manage spore load, but can not be relied upon solely to prevent late blight.”

“Late blight can be managed in an organic system, but control measures need to be proactive,” she stresses.

“In the circumstance when late blight gets out of control, early harvest and crop destruct options must be considered to limit development of inoculum that could pose heightened risk for area producers,” notes Gevens.

She, too, stresses that late blight is “a community disease.” “Management by all growers of susceptible crops is necessary,” she says.

For additional information or questions, contact Gevens with questions at 608-890-3072 or