Tomato blight is a common disease that severely damages tomato plants. It is caused by microscopic pathogens that can remain viable in the garden, leading gardeners to worry about its persistence in the soil after the growing season. This disease also affects other members of the Solanaceae family, such as potatoes and peppers. Whether tomato blight stays in the soil depends on the specific type of blight present.
Distinguishing Types of Blight
The term “tomato blight” describes two distinct diseases: Early Blight, caused by the fungus Alternaria solani, and Late Blight, caused by the oomycete Phytophthora infestans. These pathogens are biologically different, which significantly impacts their ability to survive in garden soil. Early Blight is a fungus that thrives in warm, humid conditions, typically between 75°F and 84°F (24°C and 29°C).
The Late Blight pathogen, Phytophthora infestans, is a water mold, or oomycete. It prefers cooler, moist environments, with optimal temperatures ranging between 50°F and 68°F (10°C and 20°C). This difference in classification and preferred environment dictates how each pathogen persists through the non-growing season.
How Blight Pathogens Survive in Garden Soil
The fungus responsible for Early Blight, Alternaria solani, survives readily in the soil and on infected plant debris left after harvest. It overwinters as mycelium, chlamydiospores, or conidia in the soil. Its spores can remain viable for several months, or even more than a year, without a host plant. Some research suggests this pathogen can survive for more than a decade in contaminated soil or crop residue. When spring arrives and conditions become favorable, these resting structures produce spores that are splashed by rain or carried by wind to infect new seedlings.
The survival of Late Blight, Phytophthora infestans, is generally less focused on long-term persistence directly in the soil in temperate climates. The organism typically survives the winter as asexual mycelium within infected tomato or potato tubers left in the field or in discarded cull piles. The hyphae and asexual sporangia survive for only brief periods in plant debris or soil and are often killed by frost or warm temperatures. If two different mating types (A1 and A2) are present, the pathogen can produce thick-walled sexual spores called oospores. These oospores are capable of surviving in the soil for years, though this is less common. The primary source of re-infection in the spring remains infected plant material or tubers missed during harvest.
Managing Contaminated Soil and Debris
Once blight is confirmed, immediately remove all infected plant material to reduce the pathogen load in the garden. Infected leaves, stems, and fruit must be bagged and destroyed, typically by burning or sending them to a landfill. Composting does not usually generate enough heat to reliably kill the spores of either pathogen. Reducing infected debris on the soil surface minimizes the initial source of spores for the next growing season.
Multi-year crop rotation is an effective practice for reducing the survival of both Early and Late Blight pathogens. Gardeners should avoid planting tomatoes or other susceptible Solanaceous crops (potatoes, peppers, or eggplant) in the same location for at least three years. This absence of a host plant starves the pathogen populations, allowing them to naturally decline over time. Controlling nightshade and other susceptible weeds is also important because they can harbor the pathogen and keep the disease cycle going.
Using physical barriers and careful watering techniques can prevent spores from reaching new plants, even if the soil is contaminated. Applying a thick layer of organic mulch, such as straw or shredded leaves, creates a barrier. This prevents water from splashing soil and spores onto the lower leaves of the tomato plants. Switching from overhead sprinklers to drip irrigation or soaker hoses keeps the foliage dry, which is less favorable for spore germination and spread.
For persistent contamination, especially with Early Blight, gardeners may use soil solarization. This process utilizes the sun’s heat to pasteurize the top layer of the soil. It involves thoroughly wetting the soil, covering it with clear plastic sheeting, and sealing the edges for four to six weeks during the hottest part of the summer. The elevated temperatures under the plastic can kill many soil-borne pathogens and significantly reduce the inoculum load. Alternatively, deep tilling can bury infected plant residue deeper into the soil, promoting microbial breakdown and physically removing the spore source from the surface. This is less effective than full removal.