- Late blight is a potentially devastating disease of tomato and potato, infecting leaves, stems and fruits of tomato plants.
- The disease spreads quickly in fields and can result in total crop failure if untreated.
- Late blight of potato was responsible for the Irish potato famine of the late 1840s.
Host and pathogen
Late blight is caused by the oomycete Phytophthora infestans. Oomycetes are fungus-like organisms also called water molds, but they are not true fungi.
There are many different strains of P. infestans. These are called clonal lineages and designated by a number code (i.e. US-23). Many clonal lineages affect both tomato and potato, but some lineages are specific to one host or the other.
The host range is typically limited to potato and tomato, but hairy nightshade (Solanum physalifolium) is a closely related weed that can readily become infected and may contribute to disease spread. Under ideal conditions, such as a greenhouse, petunia also may become infected.
Signs and symptoms
- Leaves have large, dark brown blotches with a green gray edge; not confined by major leaf veins.
- Infections progress through leaflets and petioles, resulting in large sections of dry brown foliage.
- Stem infections are firm and dark brown with a rounded edge.
- Firm, dark brown, circular spots grow to cover large parts of fruits. Spots may become mushy as secondary bacteria invade.
- In high humidity, thin powdery white fungal growth appears on infected leaves, fruit and stems.
- In cool, wet weather, entire fields turn brown and wilted as if hit by frost.
- Spreads most in cool (60°F to 70°F), damp weather.
- Prolonged hot dry days can halt pathogen spread.
Biology and disease cycle
Phytophthora infestans can overwinter in Minnesota if protected in potato cull piles. Overwintering in a tomato production system is unlikely but infected tomato fruits may give rise to infected volunteer seedlings the following season.
The most common routes of introduction each season are infected potato seed tubers, infected tomato transplants shipped in from other regions, or windblown sporangia (asexual spores) from the south.
Under cool, wet conditions, P. infestans can infect and produce thousands of sporangia per lesion in less than five days. These sporangia easily become air-borne, resulting in prolific spread of the pathogen.
The disease can potentially destroy entire fields in a short period of time if left unmanaged. Long-distance spread to other fields is also likely, particularly under cloudy conditions.
Even a resistant variety will show some late blight symptoms when conditions are highly favorable for disease. Levels of resistance vary between cultivars and may be more or less effective depending on the P. infestans clonal lineage present. Several resistant varieties are listed below. Check seed catalogues and the Cornell vegMD webpage for new varieties with resistance to late blight.
Tomato varieties with resistance to late blight: Mountain Magic (F1), Plum Regal (F1), Defiant PhR (F1), Mountain Merit (F1), Iron Lady (F1), Jasper (F1), Red Pearl (F1), Legend, Matt's Wild Cherry, Wapsipinicon Peach, Lemon Drop, Pruden's Purple
- Destroy potato cull piles before the growing season begins by burying them, spreading and incorporating them into fields, or feeding them to animals.
- Control volunteer potato plants, as infected plants can grow from infected tubers.
- Seed infection is unlikely on commercially prepared tomato seed or on saved seed that has been thoroughly dried.
- Inspect tomato transplants for late blight symptoms prior to purchase and/or planting, as tomato transplants shipped from southern regions may be infected.
- If infection is found in only a few plants within a field, infected plants should be removed, disced-under, killed with herbicide or flame-killed to avoid spreading through the entire field.
Fungicides are available for management of late blight on tomato. Late blight does not occur every year in Minnesota. Growers should watch for late blight symptoms in their regular scouting, particularly with weather conditions that favor disease. Reports of regional outbreaks of late blight can be found at the USAblight website.
Fungicide applications should be made prior to infection when environmental conditions favor disease to be the most effective. Phytophthora infestans is a water mold and not a true fungus. Fungicides specific to water molds must be used and applications repeated according to label instructions. For a current list of fungicides for late blight management visit the Midwest Vegetable Production Guide. Rotate fungicide groups and/or tank mix fungicides to avoid producing fungicide-resistant isolates.
Because late blight disease development is so dependent on weather, disease forecasting computer programs such as TOM-CAST have been developed to estimate when the pathogen is most active. The program uses temperature, humidity and rainfall data from a weather station. The program will determine whether a fungicide application is necessary. Following this system rather than just applying fungicide every 7 days may save several fungicide sprays per season while still providing good disease control.
Reviewed in 2016