Beginner

Scientific Name
Phytophthora infestans

Identification

• Rapid and severe defoliation occurs
• Pale green-to-brown water-soaked spots develop on the leaves; lesions enlarge rapidly and become brown to purplish-black
• A grey-to-white moldy growth may develop on the underside of infected leaves, especially under conditions of high humidity
• Brownish cankers on leaf petioles and stems may girdle and kill the plant
• Fruit lesions are firm, rough, and greasy-looking, with greyish-green or brown blotches

Often Confused With
Buckeye Rot

Period of Activity
The late blight fungus prefers temperatures from 15- 21°C (59- 70°F).  Cool nights and warm days, with moist weather, are ideal for disease development.

Scouting Notes
Key diagnostic features on foliage are lesions that are not stopped at leaf veins.  Symptomatic leaves can be sealed in a plastic bag with a damp paper towel overnight, then checked for a gray to white moldy growth on the underside.  Fruit symptoms are unlikely to be confused with other diseases, except buckeye rot.  However, late blight causes the fruit surface to become rough.

Thresholds
No tolerance.  This disease is easily spread by wind and can rapidly destroy the crop.

Advanced

Scientific Name
Phytophthora infestans

Identification
Initial leaf symptoms are pale green to brown water-soaked spots, which enlarge rapidly and become brown to purplish-black.  A pale yellow or green halo may surround the leaf lesions.  On the underside of leaves, spots may develop a grey to white moldy growth, especially under conditions of high humidity.  Brownish cankers may appear on leaf petioles and stems.  These stem lesions may quickly girdle the stem and kill it.  Under favourable conditions, the pathogen can blight the foliage so quickly, that it appears the plants were hit by frost.  Lesions on fruit appear as firm greyish-green to brown, rough, irregular shaped blotches, which rapidly enlarge.

Often Confused With
Buckeye rot (Fruit symptoms are unlikely to be confused with other diseases, except buckeye rot; however, late blight causes the fruit surface to become rough.)

Biology
Spores are produced from 10- 27°C (50- 81°F).  The fungus grows most actively from 15- 21°C (59- 70°F).  The disease may overwinter in plant debris or cull piles (potato or tomato).  The organism can travel great distances by wind. 

Period of Activity
The late blight fungus prefers temperatures from 15- 21°C (59- 70°F).  Cool nights and warm days, with moist weather, are ideal for disease development.

Scouting Notes
Key diagnostic features on foliage are lesions that are not stopped at leaf veins.  Symptomatic leaves can be sealed in a plastic bag with a damp paper towel overnight, then checked for a gray to white moldy growth on the underside.  Fruit symptoms are unlikely to be confused with other diseases, except buckeye rot; however, late blight causes the fruit surface to become rough.

Thresholds
No tolerance.  This disease is easily spread by wind and can rapidly destroy the crop.

Management Notes

• Preventative fungicide sprays for early blight, Septoria and anthracnose protect against late blight.
• If the disease is present in the area, follow a 5- 7 day fungicide schedule, using fungicides recommended specifically for late blight control.