late blight
Beginner
Scientific name
Phytophthora infestans
Identification
Leaves: Initial symptoms on leaves are light to dark-green water-soaked spots. The spots are circular to irregular in shape. These lesions are usually at the tips or margins of the leaves. Later, the lesions turn brown and are usually surrounded by a yellowish halo. In humid conditions, a white fungal growth (mycelium and spores ) develops on the underside of leaves
Petioles and Stems: Lesions on petioles and stems are black, irregular, elongated spots.
Tubers: Tuber external symptoms appear as slightly sunken, brown to purplish areas of variable size. The internal tuber symptom is a tan to brown granular dry rot. There is no clear separation between the infected and the healthy tissue.
Often Confused With
Foliage infection: Early
blight, White mold, Botrytis
grey mold, Air pollution, Frost damage, Fertilizer damage
Stem infection: Aerial soft rot
Tuber infection: Pink rot, Pythium
leak, Pink eye
Period of Activity
Throughout the season and in storage.
Scouting Notes
Scouting for late blight should begin as soon as green tissue emerges. Fields should be monitored closely, at least twice a week to detect late blight at its early stages. Walking the center pivot wheel tracks is recommended because of the long periods of high humidity that occur in those tracks.
When walking fields, stop at many sites as possible and check plants at random. Examine the lower part of the plants, stems and underside of leaves. Be sure to look for late blight in the lower portions of the plant, where the foliage stays wet longer and the disease is most likely to begin.
Areas of the field that should always be checked when scouting for late blight are:
- Low-lying areas
- Compacted areas
- Rows close to tree lines
- Field edges along creeks or ponds
- Pivot center point and pivot wheel tracks
- Weedy areas
- Any area that is protected from wind where the leaves tend to remain wet longer.
What to look for: Look for elongated, dark brown to black spots or streaks on stems. On leaves, initial lesions are dark green water-soaked areas often near the margin or tip of leaflets. Then the lesions turn dark brown.
Rubber (or disposable) pants and boots which can be washed after leaving a field, should be used if disease is present.
Thresholds
One infected plant in the entire field is enough to start an epidemic.
Advanced
Scientific name
Phytophthora infestans
Identification
Leaves: Initial symptoms on leaves are light to dark-green water-soaked spots. The spots are circular to irregular in shape. These lesions are usually at the tips or margins of the leaves. Later, the lesions turn brown and are usually surrounded by a yellowish halo. In humid conditions, a white fungal growth (mycelium and spores ) develops on the underside of leaves
Petioles and Stems: Lesions on petioles and stems are black, irregular, elongated spots.
Tubers: Tuber external symptoms appear as slightly sunken, brown to purplish areas of variable size. The internal tuber symptom is a tan to brown granular dry rot. There is no clear separation between the infected and the healthy tissue.
Often Confused With
Foliage infection: Early
blight, White mold, Botrytis
grey mold, Air pollution, Frost damage, Fertilizer damage
Stem infection: Aerial soft rot
Tuber infection: Pink rot, Pythium
leak, Pink eye
Biology
Late blight is one of the most devastating diseases of potatoes. If left uncontrolled and weather conditions favour disease development it can devastate a potato field in 5 to 7 days.
The late blight fungus survives from one year to the next on infected tubers. These can be cull tubers, tubers left in the field after harvest, slivers from seed cutting operations or infected seed tubers. When infected seed is planted, some of the seed might rot before plant emergence or some infected plants can emerge driving the disease up the stems. The fungus produces spores on the lesions of infected plants at any time during the growing season. The spores are splashed by rain to neighboring healthy plants or by wind to other areas of the field. Long distance spore dispersal is by wind. In addition farm equipment carrying contaminated plant material can spread the fungus to healthy fields
Environmental conditions conducive to disease development are:
- Temperature: below 260 C
Humidity: The fungus requires 90% or higher humidity for spore production - Temperature: below 180 C is ideal for spore germination
- Moisture: free moisture for 8-12 hours for spore germination and penetration of plant tissues.
After infection, late blight symptoms develop 3 to 4 days.
Tubers are infected in the potato hill by spores washed by water from infected foliage. Tubers are also contaminated with spores of the fungus if they are dug while infected vines still have active lesions.
Period of Activity
Throughout the season and in storage.
Scouting Notes
Scouting for late blight should begin as soon as green tissue emerges. Fields should be monitored closely, at least twice a week to detect late blight at its early stages. Walking the center pivot wheel tracks is recommended because of the long periods of high humidity that occur in those tracks.
When walking fields, stop at many sites as possible and check plants at random. Examine the lower part of the plants, stems and underside of leaves. Be sure to look for late blight in the lower portions of the plant, where the foliage stays wet longer and the disease is most likely to begin.
Areas of the field that should always be checked when scouting for late blight are:
- Low-lying areas
- Compacted areas
- Rows close to tree lines
- Field edges along creeks or ponds
- Pivot center point and pivot wheel tracks
- Weedy areas
- Any area that is protected from wind where the leaves tend to remain wet longer.
What to look for: Look for elongated, dark brown to black spots or streaks on stems. On leaves, initial lesions are dark green water-soaked areas often near the margin or tip of leaflets. Then the lesions turn dark brown.
Rubber (or disposable) pants and boots which can be washed after leaving a field, should be used if disease is present.
Thresholds
One infected plant in the entire field is enough to start an epidemic.
Management Notes
Late blight is one of the most devastating diseases of potatoes it could destroy a potato field in a few days if the weather is wet and no effective fungicides are applied. The following guidelines will help reduce the risk of a late blight infection.
- Dispose of cull potatoes and seed slivers properly by burying to a depth of 60 cm before any potatoes emerge in the spring.
- Control volunteer potato plants as they emerge.
- Plant certified seed that is free of late blight (from late-blight-free production areas).
- Do not over-irrigate. Monitor irrigations carefully.
- Do not irrigate when free moisture will stay on leaves for an extended time, i.e. try to water during late night and early morning so foliage can dry quickly during daytime hours.
- Avoid having wet spots in fields.
- Shut off the inner two towers of center pivot systems.
- Apply protectant fungicides.
- Start the first application when foliage is 15 cm high and again just before row closure.
- Apply additional fungicides according to crop growth and weather conditions.
