Potato blight refers to a pair of highly destructive plant diseases that affect the foliage, stems, and tubers of potatoes and other related plants, most notably tomatoes. While the term “blight” suggests a single condition, it actually encompasses two distinct diseases: Early Blight and Late Blight. These diseases are caused by two entirely different types of pathogens, each with unique biological properties and environmental needs, leading to varying levels of damage and different times of appearance.
Late Blight: The Oomycete Pathogen
Late Blight is caused by the organism Phytophthora infestans, famously known for triggering the Irish Potato Famine in the 1840s. This pathogen is not a true fungus but an oomycete, also known as a water mold, and is more closely related to brown algae. Oomycetes have cell walls made of cellulose, unlike the chitin found in true fungi, and their reproductive cycle relies heavily on water.
The disease is aggressive due to its rapid, asexual life cycle, which can complete in as little as five days under ideal conditions. The oomycete produces specialized spores called sporangia, which are released from the plant and spread quickly by wind and rain. When sporangia land on a wet leaf surface, they can either germinate directly or, under cooler temperatures (below 15°C), release motile zoospores. These tiny, swimming zoospores efficiently infect new plant tissue, quickly establishing a systemic infection. The pathogen’s mycelium grows between plant cells and extends feeding organs called haustoria to draw nutrients, which ultimately causes rapid tissue death.
Early Blight: The Fungal Pathogen
Early Blight is caused by the true fungus Alternaria solani, which generally causes less catastrophic damage than Late Blight. As a fungus, this pathogen has cell walls made of chitin and typically affects potato plants later in the growing season, especially when plants are older or under stress. It has a polycyclic life cycle, meaning it can produce multiple infection cycles within a single season, though its spread is slower than Late Blight.
Alternaria solani produces multicellular conidia (spores), which are dispersed primarily by wind and rain splash. The spores require a wound, a natural opening like a stomata, or prolonged leaf wetness to successfully infect the plant tissue. Once the fungus infects the host, it forms a lesion from which more conidia are released, spreading the disease to other parts of the plant or neighboring crops. While it can cause significant yield reductions, the disease rarely leads to the complete, rapid destruction of the entire crop that Late Blight causes.
Distinct Symptoms of Each Blight
Identifying the specific blight depends on recognizing the distinct visual symptoms on the potato plants and tubers. Late Blight first appears on the leaves as small, water-soaked spots that rapidly expand into large, dark brown or purplish-black lesions that are irregular in shape. In moist conditions, a characteristic white, fuzzy growth—the spore-producing structures—can be seen on the underside of these lesions. Infected tubers develop a reddish-brown decay beneath the skin that is initially firm but often leads to a foul-smelling soft rot due to secondary bacterial invasion.
Early Blight symptoms are visually different, typically starting on the older, lower leaves. The hallmark sign is the appearance of dark brown to black spots that exhibit distinct concentric rings, giving them a “bullseye” or “target spot” appearance. These spots remain relatively small and dry compared to the watery lesions of Late Blight, causing the surrounding leaf tissue to turn yellow. Tuber infection from Early Blight is less common, resulting in dark, sunken spots with a dry, leathery or corky texture underneath the surface.
Environmental Factors and Transmission
Both pathogens require specific environmental conditions to thrive, dictating when and where they pose the greatest threat. Late Blight favors cool and highly moist conditions, with optimal sporulation occurring between 12°C and 18°C and with prolonged periods of leaf wetness or humidity above 90%. The pathogen survives the winter primarily as mycelium within infected seed potatoes or in cull piles of leftover tubers. Spores are carried to new crops over long distances by the wind and locally by rain splash, which also washes spores into the soil to infect developing tubers.
Early Blight prefers warmer temperatures, with the fungus being most active between 24°C and 29°C, combined with high humidity or heavy dew. This warmer preference explains why it often appears later in the season after the plant has passed its peak growth. The Alternaria solani fungus can survive between growing seasons in infected plant debris and soil for up to three years. Transmission is primarily through the wind and water dispersal of its conidia, which spread from the soil and infected plant matter to the foliage.