Understanding and Managing Early Blight Disease of Potato

Early blight is a common foliar disease that affects potato crops worldwide This disease can significantly reduce yield and tuber quality if not properly managed. As a potato grower, it is critical to understand what early blight is, how to identify it, and strategies for effectively managing this problematic disease.

What Causes Early Blight of Potato?

Early blight of potato is caused by the fungus Alternaria solani. This pathogen can overwinter in plant debris left in fields after harvest in the soil, on seed tubers and on solanaceous weeds. The primary inoculum in the spring comes from spores produced on the overwintering fungus. These spores are spread by wind, rain splash, and insects onto potato foliage.

Once the spores land on leaves, they germinate under warm, humid conditions. The fungus penetrates the leaf tissue directly through the cuticle or through natural openings like stomata. The symptoms of early blight become visible 5-7 days after infection.

How to Identify Early Blight of Potato

The first step to good management is being able to spot early blight correctly. Here are the key symptoms to look for:

• Small dark brown or black lesions first appear on older, lower leaves.

• Lesions get bigger and are limited by leaf veins, which makes them pointy or not round.

• The most distinctive symptom is the concentric rings within the lesions, creating a bull’s eye or target spot pattern.

• Lesions may coalesce, covering large areas of the leaf.

• Leaves with heavy infection turn yellow, wither, and die.

• Stem lesions are elongated, black lesions on stems and petioles.

• Tuber symptoms appear after harvest as dark, sunken lesions with raised borders.

Conditions Favoring Early Blight Development

Several environmental factors influence the development and spread of early blight disease:

• Warm temperatures – The optimum temperature for infection is around 68°F.

• High humidity and prolonged leaf wetness – Free moisture is required for spore germination and penetration. Overhead irrigation and frequent rainfall promote disease spread.

• Age of plants – Older, senescing foliage is more susceptible than young growth.

• Potato variety – Earlier maturing varieties tend to be more susceptible.

• Stressed plants – Factors like poor fertility, drought, insect damage, etc. increase susceptibility.

Impacts of Early Blight Infection

If early blight is left unchecked, impacts on potato yield and quality can be significant:

• Premature defoliation due to extensive leaf lesioning reduces photosynthetic capacity.

• Smaller, fewer tubers are produced due to impaired photosynthesis.

• In storage, tuber lesions enlarge and result in shriveling.

• Fresh market quality is reduced by poor appearance of infected tubers.

• Processing quality suffers from decay and blemishes on tubers.

Integrated Management of Early Blight

The most effective way to control early blight is to use both cultural practices and targeted fungicide applications together.

Cultural Control Methods

• Use clean seed tubers free of the early blight pathogen. Avoid saving your own seed tubers.

• Rotate potato fields to a non-host crop like corn or soybean for 2-3 years to reduce pathogen levels.

• Eliminate cull piles and volunteer potatoes, which harbor the fungus.

• Control solanaceous weeds like nightshade which can be alternate hosts.

• Plant later maturing, longer season potato varieties which are less susceptible.

• Maintain proper fertility and irrigation to avoid plant stress.

• Dig vines promptly after harvest to speed decomposition of foliage.

Fungicide Applications

Foliar fungicides are the best way to keep early blight from happening. These provide recommendations for fungicide use:

• Begin scouting fields regularly at row closure for early symptoms.

• Base spray programs on an early blight forecasting system using P-days or disease severity values.

• Initiate fungicide applications at 300 P-days or when early symptoms are found.

• Apply protective fungicides like mancozeb, chlorothalonil or fluopyram on a 7-10 day schedule.

• Alternate different mode-of-action fungicides to minimize resistance development.

• Increase spray frequency during periods of rapid disease development.

• Cover undersides of leaves for best results.

Early blight is a challenging but manageable disease in potato production. Careful monitoring to detect initial symptoms, avoidance of stress, and properly-timed fungicide programs provide the best opportunity to minimize yield losses and protect potato quality threatened by this disease. Integrating cultural practices and fungicides that utilize multiple effective modes of action is key to success.