Plant blights pose common challenges for gardeners, often causing significant damage to various crops. Recognizing the distinct symptoms and favored conditions of different blight types is important for effective plant health management. This article clarifies the differences between early blight and late blight, two prevalent plant diseases.
Early Blight Characteristics
Early blight is caused by the fungus Alternaria solani, affecting potatoes, tomatoes, and related plants. Symptoms typically first appear on older, lower leaves as the plant matures. These initial spots are usually small, dark, and circular, often developing concentric rings that create a “bullseye” pattern, sometimes surrounded by a yellow halo. As the disease progresses, these spots enlarge and merge, leading to leaf tissue destruction and eventual leaf drop.
The fungus thrives in warm, humid conditions, with temperatures between 75°F to 85°F (24°C to 29°C), and requires prolonged periods of leaf wetness from dew or rain. Stress factors, such as poor plant nutrition, drought, or heavy fruit loads, can increase a plant’s susceptibility. The pathogen can also infect stems and fruits, causing sunken lesions on stems and dark, firm spots with concentric rings on the fruit.
Late Blight Characteristics
Late blight is a destructive disease caused by the oomycete Phytophthora infestans, often referred to as a water mold. This pathogen primarily attacks potatoes and tomatoes, but can also affect related plants like petunia. Initial symptoms on leaves are typically small, water-soaked lesions that quickly expand into large, irregular gray, pale green, or dark brown blotches. Under moist conditions, a fuzzy, whitish mold-like growth may appear on the undersides of infected leaves, especially at the margins of the lesions.
Unlike early blight, late blight can appear on any part of the plant, including younger leaves and stems, and spreads rapidly. Stem lesions are often dark brown or purplish-black. Infected plants can wilt and die quickly, sometimes within a week to two weeks under favorable conditions. The disease favors cool to moderate temperatures, between 59°F and 80°F (15°C and 27°C), with high humidity and prolonged periods of leaf wetness.
Differentiating the Blights
Distinguishing between early and late blight involves observing several key characteristics. Early blight is caused by Alternaria solani, a true fungus. Late blight is caused by Phytophthora infestans, an oomycete, which is a fungus-like microorganism. This difference in pathogen type contributes to their varying behaviors and environmental preferences.
Symptom appearance offers clear distinctions. Early blight lesions on leaves typically feature a characteristic “bullseye” pattern with concentric rings and often a yellow halo, appearing first on older leaves at the base of the plant. In contrast, late blight lesions are initially water-soaked and irregular, rapidly turning dark brown or purplish-black. They frequently exhibit a fuzzy white growth on the undersides of leaves.
Environmental conditions are another differentiator. Early blight prefers warmer temperatures and humid conditions with dew, worsening when plants are stressed. Late blight, however, thrives in cooler, wet conditions with prolonged periods of leaf wetness. Speed of spread also differs; early blight progresses more slowly, while late blight can devastate entire plants or fields within days due to its rapid spread by wind-dispersed spores.
Managing Blight Diseases
Managing both early and late blight largely focuses on preventative measures and good garden hygiene. Cultural practices reduce infection risk and spread.
Proper spacing between plants promotes air circulation, which helps foliage dry quickly and reduces the duration of leaf wetness, conditions conducive to disease development.
Watering plants at their base, rather than overhead, minimizes splashing soil onto leaves and keeps foliage drier, which can prevent spore germination and spread.
Remove and properly dispose of infected plant material; do not compost diseased plant debris, as pathogens can survive and reinfect new crops. Crop rotation helps break the disease cycle by limiting the availability of host plants for pathogens in the soil. Selecting resistant plant varieties also significantly reduces susceptibility to these diseases.