Alternaria Brown Spot
Beginner
Scientific Name
Alternaria alternata
Identification
The leaves in the middle of the plant are most susceptible to brown spot and they show the highest incidence of the disease. Initial lesions are tiny brown circles or “pepper spots”. Older lesions of brown spot are round with concentric rings that are not as noticeable as the rings of early blight lesions
Brown spot lesions may coalesce to form larger necrotic areas with dark-brown margins. These necrotic areas usually form near the edges of leaflets. Brown, elongated small lesions may form on stems when disease pressure is high.
Tubers may be infected at harvest. Infection results in small black holes in the tubers (black pit). These holes are similar to pits caused by common scab but are deeper, narrower and blacker.
Often Confused With
Early
blight
Air
pollution
Period of Activity
It is more prevalent in warm seasons. It usually develops after the rows close.
Scouting Notes
Fields should be monitored at least twice a week. When walking fields, stop at many sites as possible and check plants at random.
Thresholds
Field scouting should determine the initial development of the disease. Then growers can make management decisions.
Advanced
Scientific Name
Alternaria alternata
Identification
The leaves in the middle of the plant are most susceptible to brown spot and they show the highest incidence of the disease. Initial lesions are tiny brown circles or pepper spots. Older lesions of brown spot are round with concentric rings that are not as noticeable as the rings of early blight lesions
Brown spot lesions may coalesce to form larger necrotic areas with dark-brown margins. These necrotic areas usually form near the edges of leaflets. Brown, elongated small lesions may form on stems when disease pressure is high.
Tubers may be infected at harvest. Infection results in small black holes in the tubers (black pit). These holes are similar to pits caused by common scab but are deeper, narrower and blacker.
Often Confused With
Early
blight
Air
pollution
Biology
Alternaria alternata persists in plant refuse in the soil on weeds and on other susceptible hosts such as cucurbits and brassicas. Infection occurs when windborne spores germinate on potato foliage. Brown spot is not considered as damaging as early blight nor as aggressive. High humidity, leaf wetness such as prolonged dew, and warm temperatures are conducive to infection.
Period of Activity
It is more prevalent in warm seasons. It usually develops after the rows close.
Scouting Notes
Fields should be monitored at least twice a week. When walking fields, stop at many sites as possible and check plants at random.
Thresholds
Field scouting should determine the initial development of the disease. Then growers can make management decisions.
Management Notes
Management of brown spot requires an integrated program of cultural practices which include the following strategies:
- Crop rotation for at least 2-3 years.
- Avoid crop stress:
- Provide adequate levels of nutrients
- Manage other pests
- Match irrigation with crop needs. Do not over-irrigate.
- Harvest tubers when the skin is set and avoid mechanical injury.
