Difference Between Early And Late Blight

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Early blight and late blight, two serious diseases of potato, are widely distributed. Both are found everywhere potatoes are grown. The terms “early” and “late” refer to the relative time of their appearance in the field, although both diseases can occur at the same time.

Late blight is caused by Phytophtora spp, whereas early blight is caused by Alternia spp. The most apparent differences are how they appear on your plant. In this article find an overview of the two disease including how to remedy or get rid of them.

Late Blight

Late blight of potato and tomato is caused by the fungus  Phytophthora infestans. Although late blight can occur at any time during the growing season, it is more likely to be seen during cool wet seasons. The disease can spread rapidly during cool, rainy weather, killing plants within a few days. Daytime temperatures between 15 – 21 °C, night temperatures between 10 – 15 °C, and relative humidity near 100% create ideal conditions for infection and spread of the disease. The fungus becomes inactive during dry periods. 

The late blight fungus survives in infected potato tubers in the ground or in cull piles and in infected tomato fruits and crop debris. The fungus can also survive in perennial weeds, such as nightshade. As infected tubers and perennial weeds germinate and grow, the fungus becomes active and reproduces on the young plants. 

Late blight symptoms can develop on leaves, stems, branches, and in case of tomatoes on both green and ripe fruits. In potatoes, tubers can also be infected. On leaves, pale green to brown spots, sometimes with a purplish tinge, appear on the upper surface of leaves.

Leaf spot margins often are pale green or water-soaked. The spots may enlarge rapidly until entire leaflets are killed. In moist conditions, a downy white greyish mould usually develops near the margin of leaf spots on the underside of leaves. In dry weather, affected foliar parts may appear dry and shrivelled. Stems can also develop elongated, greyish watery brown lesions. 

On tomato fruit, grey green watery spots can develop on the upper half of the fruit, which later spread and turn greasy brown and bumpy. In moist weather, a white downy fungal growth may appear on the affected fruit-rot surface. Infected potato tubers exhibit wet and dry rots.

Early Blight

Early blight is caused by the fungus, Alternaria solani, which survives in infected leaf or stem tissues diseased potato tubers on or in the soil and in infected tomato fruits. This fungus is universally present in fields where susceptible crops have been grown. It can also be carried on tomato seed and in potato tubers. 
Spores are formed on infected plant debris at the soil surface or on active lesions over a fairly wide temperature range, especially under alternating wet and dry conditions. 

Infection occurs in warm, humid weather with heavy dews or rain. Periods of warm rainy weather, with temperatures between 21-24 °C favour outbreaks of early blight.

Seriousness of early blight is dependant on weather conditions and crop variety. Early blight can develop quite rapidly under humid warm conditions and is more severe when plants are stressed by poor nutrition, drought, nematode attack or a heavy fruit load. 

Spots begin as small, dark, dry, papery flecks, which grow to become brown-black, circular-to-oval areas. The spots are often bordered by veins that make them angular. The spots usually have a target appearance, caused by concentric rings of raised and depressed dead tissue. A yellowish or greenish-yellow ring is often seen bordering the growing spots.

As the spots become very large, they often cause the entire leaf to become yellow and die. This is especially true on the lower leaves, where spots usually occur first and can be very abundant. The dead leaves do not usually fall off. Dark brown to black spots can occur on stems.

Typical fruit spots occur at the stem-end as a rot that radiates out from the area of attachment between the calyx and the fruit. The spot is usually brown to black, firm, depressed and has distinct concentric rings.

Early Blight vs Late Blight: Key Difference

Basis of ComparisonEarly BlightLate Blight
Causal OrganismsCaused by the fungus Alternaria solani.Caused by the oomycete Phytophthora infestans.
Pathogen TypeFungus (Ascomycete).Oomycete (similar to algae and fungi but with different cell walls and reproductive structures).
SymptomsCircular lesions with concentric rings and dark centers on leaves. Lesions may have a target-like appearance.Irregularly shaped, water-soaked lesions with a pale green to grayish appearance, often surrounded by a yellow halo.
Leaf InfectionTypically starts from lower leaves and progresses upwards.Can start from any part of the plant and spread rapidly, affecting both lower and upper leaves.
Spore Production and DispersalProduces spores (conidia) on lesions, which are easily spread by wind and rain.Produces spores (sporangia) on infected plant tissue, which are dispersed by wind and can cause secondary infections.
Environmental ConditionsThrives in warm and humid conditions.Thrives in cool, wet conditions, and is more severe during periods of high humidity.
ManagementCultural practices like crop rotation, pruning, and maintaining good air circulation can help. Fungicides can also be used.Removal of infected plant parts, crop rotation, and fungicides are important. Late Blight is more challenging to manage due to its rapid spread.
Economic ImpactGenerally, causes less severe damage compared to Late BlightHistorically responsible for devastating crop losses, including the Irish Potato Famine in the 1840s.

Key Takeaways

  • Early blight of potato is caused by the fungus, Alternaria solani, which can cause disease in potato, tomato, other members of the potato family, and some mustards. 
  • Late blight of potato is a serious disease caused by Phytophthora infestans. It affects potato, tomato and, occasionally, eggplant and other members of the potato family. 
  • Early blight is caused by a fungus that overwinters as spores and mycelium in infected crop residue.
  • Late blight is caused by a fungus-like microorganism that can overwinter on crop residue.
  • Early blight, as the name implies, appears before the onset of late blight but does no spread throughout the foliage.
  • Lesions caused by late blight can be found anywhere on the plant, but are mostly found on the new growth whereas early blight typically starts on the lower leaves and slowly moves up the plant. Also, lesions caused by late blight tend to be light brown or tan in color while lesions caused by early blight tend to be dark brown in color with concentric rings.
  • The symptoms of EB start out as small, dark spots. As the spots get larger, they have definite concentric rings, looking like a target. The tissue around the spots is usually yellow. The spots will grow and eventually fuse together, forming irregular dead patches on the leaves. The stems develop brown, dry, sunken lesions.
  • The symptoms of EB start out as small, dark spots. As the spots get larger, they have definite concentric rings, looking like a target. The tissue around the spots is usually yellow. The spots will grow and eventually fuse together, forming irregular dead patches on the leaves. The stems develop brown, dry, sunken lesions.