Potato spot is a general term for various lesions and discolorations on potato leaves, stems, and tubers. These diverse symptoms can make identification challenging, but understanding them is important for effective management.
Identifying Common Potato Spots
Early Blight
Early blight appears as circular to angular dark brown spots on older, lower leaves. These spots often feature concentric rings, resembling a target pattern. A yellowing of the surrounding leaf tissue may occur, and infected leaves can turn yellow and drop off as the disease progresses. Elongated, dark brown to black lesions may also develop on stems.
Late Blight
Late blight manifests as irregular, water-soaked lesions on younger leaves and stems. These spots quickly expand, becoming dark brown to purplish-black and often appearing greasy. Under humid conditions, a fuzzy white mold-like growth can be observed on the underside of infected leaves, particularly at the lesion edges. This disease spreads rapidly, leading to the collapse and decay of the entire plant.
Bacterial Spot
Bacterial spot presents as small, angular, water-soaked lesions on potato leaves. These spots often turn dark and necrotic, sometimes developing a yellow halo. Their angular shape is typically due to limitation by leaf veins. While primarily observed on leaves, these lesions can also affect stems. Severe cases can lead to significant leaf loss.
Causes and Contributing Factors
Potato spots are caused by specific pathogens. Early blight is caused by the fungal pathogen Alternaria solani, while late blight is attributed to the oomycete (fungus-like organism) Phytophthora infestans. Bacterial spot is caused by various bacteria, such as Xanthomonas campestris pv. vesicatoria.
These pathogens thrive under specific environmental conditions. Alternaria solani favors warm temperatures (68°F-86°F) and leaf wetness from dew, rain, or irrigation; alternating wet and dry periods also encourage spore spread. Phytophthora infestans (late blight) prefers cool, moist conditions (50°F-78°F) and high humidity (above 90%), with prolonged leaf wetness being a significant factor in its rapid spread.
Bacterial spot pathogens, like Xanthomonas campestris pv. vesicatoria, also thrive in warm temperatures and high humidity, requiring free water for infection. These conditions allow bacteria to enter plants through natural openings or wounds. Poor air circulation, plant stress, and nutrient deficiencies increase susceptibility.
Prevention Strategies
Proactive measures are effective in preventing potato spots from emerging or spreading. Proper plant spacing allows for good air circulation, which helps to dry foliage quickly and reduce periods of leaf wetness. Ensuring adequate drainage in the soil prevents waterlogging, which can create favorable conditions for pathogens. Crop rotation is another beneficial practice; avoiding planting potatoes in the same area year after year helps to break disease cycles, as many pathogens can survive in soil or plant debris.
Selecting resistant potato varieties can significantly reduce the likelihood and severity of infection. Removing and properly disposing of plant debris after harvest helps to eliminate overwintering sites for pathogens. Avoiding overhead watering, especially late in the day, keeps foliage drier and minimizes conditions conducive to disease development. Instead, consider drip irrigation or watering at the base of the plants to keep leaves dry.
Management and Treatment
Once potato spots are observed, timely management is important to limit their impact. For organic approaches, promptly removing and destroying infected plant parts, such as leaves or stems, can help contain the spread of disease. Maintaining good garden sanitation by clearing plant debris reduces pathogen inoculum. Organic fungicides, like copper-based sprays, can offer some protection, particularly when applied preventively or at the first sign of disease. Products containing Bacillus subtilis are also available as biological control agents to suppress certain fungal and bacterial diseases.
For chemical treatments, various fungicides are available, often categorized as protectant or systemic. Protectant fungicides form a barrier on the plant surface, preventing infection, while systemic fungicides are absorbed by the plant, offering internal protection. Common active ingredients include chlorothalonil and mancozeb, which are often used for blights. It is important to always follow label instructions precisely regarding application rates, timing, and safety precautions. An integrated pest management (IPM) approach, combining cultural practices, resistant varieties, and judicious use of treatments, offers the most sustainable strategy for managing potato spots.