A Primer on the disease Late Blight

Facts About the Disease

Late blight, a disease caused by the water mold Phytophthora infestans, has been infecting potatoes, tomatoes, and perhaps eggplant for hundreds of years. The visits of this Central American native used to be fairly rare, which was fortunate, since it truly lives up to its name ("devastating plant destroyer" in Latin). It was late blight that destroyed the Irish potato crop in the late 1840s, causing at least a million Irish deaths, and forcing the migration of an even larger number to America.

Until recently, late blight outbreaks tended to be cyclic: occurring for a year or two, then going away again. In the past decade, though, the disease has become a constant in more and more areas of the United States (and probably most of the agricultural world). It had hit this corner of the United States (the Maritime Pacific Northwest) for five years in a row (1993-1997), but our excellent 1998 summer seems to have given us a respite.

Phytophthora infestans reproduces asexually by producing spores, which can only survive on living plant material. The disease is believed to live through the winter on perennial solanaceous plants (the nightshades) and on infected potato tubers. Even though the disease does not move through an infected plant's tissues, its spores can be carried by water or wind from the leaves down through the soil onto the tubers. So although these spores cannot live in the soil directly, unharvested infected potatoes which then "volunteer" the next spring can carry on the disease cycle.

It is an open question whether we will see a soil-dwelling form of late blight soon. There are two main mating types of this water mold, A1 and A2 (these are further divided up into a significant number of races, which are identified by US-1, US-8, US-11, and so on depending on the country where they were first isolated). Up until recently we had only been dealing with the races which comprise mating type A1 of the disease. Incidentally, this was the same strain that caused the Irish Potato Famine. Now the more virulent A2 mating type has been observed in the United States, and is even the dominant strain in some areas such as Idaho and southeastern Washington state. If both types come in close contact, they will be able to reproduce sexually, resulting in an egg called an oospore. Oospores are much better protected than regular spores, and can survive in the soil for an undetermined amount of time. However, so far there have been no documented cases of late blight oospores being able to survive over winter in soils outside of their Mexican homeland.

Progression of the Disease

This disease can spread incredibly fast; but it needs damp conditions to propagate. Late blight requires 24-72 hours to infect a plant. But it spreads amazingly well through the air, and can infect plants up to 30 miles away. It can take hold on one plant, and wait for conditions to "improve" to spread further.

In my garden, the determinate tomatoes are invariably hit first. They are bushier plants, which makes it harder for air to circulate; so they often stay damp in the middle for quite a while. Once the disease has successfully established itself, it then spreads to the indeterminate tomatoes and the potato plants (once weather permits). Eggplant may also be succeptible, judging by the last time I grew that vegetable.

Additionally, in this corner of the world our weather can easily stay damp for a week or two, which is all the disease needs to infect and kill the plant.

Identifying Late Blight

With both tomatoes and potatoes, late blight often starts with infection of the leaves. Early on they may yellow on the edges, or sometimes in the middle. In these early stages, it can resemble Alternaria (early blight); the main distinguishing characteristic between the two being that Alternaria's lesions will not spread across the leaf's veins, while late blight's lesions will. The disease spreads outward from the initial point of infection, with the leaf turning brown at that spot, with yellow around it. Sometimes the leaves will appear almost water-soaked, due to the fungal hyphae breaking down the leaf's cells. Usually at some stage there are brownish-black patches on the stems, often around the leaf axils; but the stems stay intact until the plant withers. Sometimes a whitish "mold" is visible on the undersides of the leaves. If late blight infects the fruit, they develop a characteristic known as firm rot: a brownish area, smooth or sometimes corrugated, that is still hard to the touch (well, as hard as a tomato ever is). If that tomato ripens, the area immediately around the brown patch will usually remain green.

Infected potato plants appear much the same. The University of Idaho has some photographs of blight-infested potatoes.

Controlling Late Blight

There is no magic bullet for late blight. Good garden cleanup, crop rotations, and removing diseased plant material will all help; but the spores are so mobile that there are no guarantees. Removing all weedy varieties of nightshade is a good idea. It is generally recommended that you buy seed potatoes that are certified as disease-free for planting (unfortunately, the legal translation of "disease-free" in the United States is "having an infection rate of 1% or lower"). If you save your own potatoes for seed, separate the seed immediately after harvest; keep them dry; and donít use any that have lesions or dark areas on the surface.

There are some additional cultural practices that are recommended for potatoes. First, cut the vines below the soil line (and remove them from the garden) a couple weeks before you dig the potatoes. Second, stop watering your potato bed once the vines have flowered (which is roughly when the potatoes are sizing up). These practices mimimize the possibility of exposing the tubers to live fungal spores (Incidentally, not watering the potatoes also enhances their storage life by encouraging the development of tougher skin). It is also a good idea to pull any volunteer potato plants as soon as they appear, to minimize the possibility of infected plants spreading the disease.

As I noted above, this disease can move incredibly fast. Fortunately, dry sunny conditions can slow or halt the progression of the disease. Often most of a planting can be saved if conditions improve, and you are willing to be ruthless about removing infected plants. Sometimes even infected plants can be saved, if they are not too far gone. I have managed to nurse LB-infected indeterminate tomato vines through to late September, by getting a cover over them (or praying for sunshine!) and severely pruning out all the infected tissue. If you miss any infected part, LB can be guaranteed to spread as soon as conditions permit.

In general it is not a good idea to compost plants which are infected with late blight, since the disease can continue to spread until all parts of the plant are dead. Throw them in the trash, or burn them (one commercial seed potato farm recommends disposing of ALL Solanaceous plant material by burning, whether it is visibly infected or not!). If you do decide to compost them (and I am not recommending it), make sure you have a good hot compost pile, and put the infected plants into the hottest part of the pile.

Currently there are no tomato varieties resistant to the blight, but some potatoes exhibit varying degrees of resistance. If you garden in an area prone to late blight, your best bet is to erect some sort of cover over the plants, like a hoop house (essentially a giant cloche) or an open-ended greenhouse. Copper fungicides will also work, if applied diligently before late blight has infected the plant.


References on Late Blight and other Diseases

Keith Mueller's The On-line Tomato Vine is a great resource for the tomato grower. It has, among other things, a step-by-step diagnostic page for tomato problems and diseases.

The American Phytopathological Society has an on-line short course called "Late Blight of Potato and Tomato". It covers the life cycle of this water mold in greater detail than this FAQ does.

The University of Idaho has a late blight page that is focused more on potatoes (no surprises there!).

I have written an article describing how to build a PVC hoophouse, which is the basic method of control I use. PVC Hoophouses have the additional benefits of providing extra heat and adding a few weeks to either end of your growing season.


Special thanks to Keith Mueller, who might've finished his thesis on early blight sooner if he hadn't been pestered by all my questions on late blight!


All contents © Travis Saling
This page was last updated November 19, 2013