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Late Blight, Early Defenses

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Early Defenses Against Late Blight

Late Blight Foliar. Courtesy oregonstate.eduA few malignant-looking spots on the leaves and stem, perhaps a smattering of white fuzz beneath the leaves, and the bottom of your stomach drops out. The first signs of late blight. As a grower of nightshades, what do you need to know to come prepared for this silent ambush before late summer hits?

Late blight is a fungal disease, perhaps known best among non-farmers and non-gardeners for causing the Irish Potato Famine. It gives tomato and potato growers a major headache every few years, taking off in response to a particular set of weather conditions. Eggplant and peppers, though also members of the nightshade family, don’t show as much susceptibility. The first signs of the disease are often dark, water-soaked, irregularly-shaped spots half an inch to an inch in diameter appearing on the leaves. A fuzzy white mold covers the spots on the leaves’ undersides.

The spots grow rapidly, eventually defoliating the plant. The infection spreads to leaf stalks and the main stem, and fruits and tubers may then be affected. On fruit, the sign of infection is a golden to brown lesion with spots that can appear sunken and rippled.

No tomato variety is immune to late blight, but breeders are constantly trying to outwit it with resistant cultivars. One of the problems with this, though, is that late blight evolves its own counter-resistance over time.

According to the National Gardening Association, Johnny’s Seeds of Maine ( just came out with a new resistant variety that looks like it may be pretty effective. Known as ‘Defiant PhR’ it also shows some resistance to early blight, also a fungal disease. Weighing in at a solid 6-8 ounces, deep red and good for slicing, it was bred to inherit major genes for late blight resistance.

Any new weapons against the common pathogen are promising for most gardeners, who’d probably prefer to avoid the fungicides favored by most conventional large-scale growers. Prevention and early eradication are among the few defenses left to those who want to grow organically (as well as copper-based sprays, but many small gardeners usually don’t have the scale to make this worthwhile).

Late blight is caused by the pathogen Phytopthera Infestans, which was likely introduced to the U.S. from Mexico in the mid-nineteenth century, making its way from there to Ireland. Worldwide spread began around the early twentieth century.

The fungus reproduces by sporangia, which disperse to healthy tissues via rain splash and wind currents. The sporangia can travel up to 40 miles airborne, which may help explain why such large geographical areas are affected within the same season.

The threat posed by P. Infestans became more formidable in the early 1990s, when several exotic strains arrived from Mexico, demonstrating increased severity on tomato and potato crops and more aggressive progression of the epidemics. Some of these strains also manifested the potential ability to reproduce sexually, creating not just new progeny with the same DNA, but recombinant individuals with unpredictable genotypes. Spores from these strains would be able to survive in the absence of living plant tissue as a host. (The need for such a host imposes healthy season-to-season limits on the survival of strains we’re used to.)

Blight on Tomato. Courtesy cornell.eduUnder conditions favorable to late blight, epidemics can spread much more rapidly in tomatoes than potatoes. Once sporangia disperse, they can germinate within a few hours if they have the proper moisture, but can also survive dry conditions for a few hours. Infections then become visible as lesions after a few days. Conditions that best favor disease development are 60-80 degree days with slightly cooler nights, and wet conditions. Epidemics are rapid and obliterating. The pathogen has an extremely high reproductive potential and spread generally seems to happen at a rapidly accelerating rate – the more plants become infected, the more spores are produced, and the more plants that are subsequently infected by these spores, a cycle that quickly becomes self-reinforcing. Perhaps you could say this about the propagation of any species, but P. Infestans seems to manage it at a particularly alarming rate.

Since sporangia need a living host between seasons, they often survive in infected potato tubers left in the soil or discarded during harvest. These survive in the soil to become volunteers the following year.

Basic measures like sanitizing tomato-growing equipment like stakes and cages is therefore important, as well as keeping a several-year rotation in a given area for tomatoes and potatoes. Using cultivars with some resistance ability is also a good way to slow the pathogen growth rate.
Carefully water only the crops’ root zone, rather than the foliage. Drip-tape irrigation is a good way to do this easily and efficiently.

Hilling potatoes gives tubers an extra buffer against sporangia that may land on the soil surface, provided heavy rains don’t wash it quickly down into the soil.

Inspect crops to catch symptoms early so you can try to isolate the problem – especially low spots in the field, spots with dense foliage, shaded or wet areas, or crops next to woods or hedgerows. If only a few plants are affected, destroy them. Burn them, bury them more than two feet deep, or make a pile of the infected plants and cover them with dark plastic or a tarp and leave them in the sun until they are completely dead, then dispose of them in a garbage bag. (Don’t compost them.) This helps to remove the threat of spread to other nearby growers. Freeze kills during winter are usually enough to kill the living plant host as well.

Very dry or hot weather during both day and nighttime may temporarily stall the epidemic, but stem infections are much more resistant to drying and can often wait to sporulate until enough moisture returns.

While tomato seed has never proven to be a problem from year to year, potato seed should not be saved for planting the following year. Use only health-certified potato seed. Some potatoes with resistance (again, not immunity) to late blight include Elba, Kennebec, Sebago, Allegany, and Rosa.

In many parts of the Northeast, farmers and gardeners are now carefully nursing their tomato transplants and preparing the ground to receive them. The scheduled date to put them in the ground may be right around the corner, but the threat of late blight seems comfortably distant. It’s a hallmark of the season’s timeline; don’t let it sneak up on you this year!

– Genevieve Slocum is a Contributor to The Free George.

The Free George is the online magazine and visitors’ guide of Upstate NY, covering things from Albany to Lake Placid, including Saratoga, the Lake George region and the Adirondacks. Check out our new City Blogs section for our extended coverage areas as well.

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