Late blight is a plant disease that affects a variety of Solanum spp., including eggplant, pepper, nightshade weeds, and petunia. However, it is most infamously known for its destruction of potato, S. tuberosum, and tomato, S. lycopersicum.
Symptoms of late blight may be found on any above-ground part of the tomato plant. Infected leaves typically have green to brown patches of dead tissue surrounded by a pale green or gray border. When the weather is very humid and wet, late blight infections can appear water-soaked or dark brown in color, and are often described as appearing greasy. White, fuzzy growth may be found on the undersides of leaves or on lower stems. Stem and petiole lesions are brown and are typically not well defined in shape. Discoloration may also occur on the flowers, causing them to drop. Symptomatic tomato fruits appear mottled, often with golden to dark brown, firm, sunken surfaces. White, fuzzy pathogen growth can also be found in association with the fruit lesions.
Disease spread and conditions
Phytophthora infestans is the oomycete, or water mold pathogen, responsible for tomato late blight. In the united states, this fungus-like organism overwinters primarily in plant debris as mycelia, a filamentous thread-like growth of the pathogen. In some countries of Europe and South America, the pathogen can produce a soil-persistent spore known as the oospore, but this spore has not been identified in the United States. The pathogen spreads by movement of asexual spores: sporangia (airborne spores) and zoospores (water-swimming spores). The pathogen invades host plant cells causing plant death.
The production of pathogen spores is promoted by moist conditions (90-100% relative humidity) with moderate temperatures (60-80°F). Sporangia may germinate at 64-75°F, and zoospores are released at 46-64°F. Mycelia prefer temperatures around 73°F for optimal growth. Wisconsin residents can refer to the disease prediction and weather models available at Vegetable Disease and Insect Forecasting Network (VDIFN) website.
Phytophthora infestans reproduces asexually (via sporangia and zoospores). In warm weather, sporangia land on susceptible host tissue and directly germinate to create an infection. In cooler wet weather, however, sporangia produce motile zoospores that can infect host tissues. After just 4-5 days of sufficient moisture and moderate temperatures, a new phase of sporulation can occur at the site of the initial infection. New sporangia are then carried by wind and splashed water to new plant tissue, creating new infections. And the disease spreads. Continued and rapid repetition of this asexual disease cycle causes large-scale and fast-advancing late blight epidemics.
A disease severity predictive model based on air temperature and relative humidity is available at the Vegetable Disease and Insect Forecasting Network (VDIFN) website. From the Disease tab, select the “Late Blight” model. This model uses Disease Severity Values (DSVs) computed from the last week of gridded NOAA weather data to calculate the risk of Phytophthora infestans development, which is displayed as a colored map overlay. The start point should be adjusted to crop emergence or the last fungicide spray, whichever is more recent. The model takes into account the total DSVs accumulated during a season, as well as the amount accumulated since the start date. Click any grid point in VDIFN to get more detailed weather and disease progression information for that location.
Strategies for managing late blight in tomato include planting resistant cultivars, eliminating volunteers (tomato plants that have re-seeded from the previous year’s crop), spacing plants to increase airflow and reduce humidity, and applying preventive and effective fungicides to avoid infection.
Various fungicides are registered for use in controlling tomato late blight. Some categories of fungicides are appropriate in conventional systems, and others are appropriate in organic systems. For either production system type, it is critical to apply fungicides before initial infection for best control of late blight.
Commercial producers in Wisconsin have adopted use of Blitecast. This disease-forecasting tool indicates appropriate timing for preventive fungicide applications based on favorable weather. This tool can aid in reducing the need for multiple, calendar-based fungicide applications prior to actual disease risk. For Wisconsin- specific fungicide recommendations, click here for a list of fungicides registered for commercial use, or refer to Commercial Vegetable Production in Wisconsin (A3422), a guide available through the UW Extension Learning Store website. Or, for home garden fungicide recommendations, see Home Vegetable Garden Fungicides (XHT1211), a fact sheet available through the UW Plant Disease Diagnostic Clinic website.
Adapted from UW Learning Store publication A4052-01, written by Amanda Gevens and Jaimie Wilbur, 2014. Download PDF. See also the UW Plant Disease Diagnostic Clinic fact sheet on late blight.