Phytophthora infestans, fungus; foliar = late blight


Late blight has changed dramatically in recent years. Symptom descriptions are based on US strain 1, an A1 mating type, which has been the only strain in the US for over a century until the 1990s. This strain has been nearly totally replaced since 1993 by US strain 8, an A2 mating type. This new strain is much more virulent and in many ways acts quite differently from the old strain #1. US-8's vine attack is beginning to be understood, but even less is understood about its soil leaching or tuber behavior. The tuber symptoms described here are still largely based on US-1.

Tuber infection occurs by spores leaching through the soil after being washed off infected foliage. Some infection may occur during harvest (lifting) by direct exposure to infected, still living vines. Late blight needs living tissue to survive (unless/until A1 and A2 type strains meet and mate). Currently, it is presumed that late blight can spread in storage.

Early Symptoms:

The tuber's skin has patchy and irregular, brown to purple areas. When cutting through these patches, the inside is a mahogany (reddish brown), firm rot progressing to a quarter to a half-inch deep. This rot is brown with a very irregular border "spiky" or "toothy."

Later Symptoms:

Skin patches darken and sink into the tuber. The below skin rot is granular and may have dark projections going deeper into the tuber. Late blight may be confused with pink eye. Tuber breakdown is due to secondary, saprophytic bacterial infections, "wet rot phase." Bacterial soft rot and Fusarium rot can also invade.

True identification of late blight on tubers requires laboratory testing.

Control Practices:

Treat foliar late blight with registered fungicides in a treatment program. Delay harvest until vine kill is complete. There is current research which suggests that soil treatment with fixed coppers or chlorothalonil may kill spores in and on soil. Sulfuric acid may kill spores at the surface if ground isn't too wet or spores aren't covered by soil, debris or vine. Keep cull piles well away from storage facilities. A lot of research is needed to deal with this new threat.


Tuber Blemishes