Late blight-infected tubers have brown, dry, sunken lesions that cover granulated brown or tan tissue up to 12 inch deep. Secondary infections, caused by bacteria or other fungi, can result in the slimy breakdown of additional tuber tissue in storage.

Persistence and Transmission. Late blight most commonly occurs under cool, wet conditions. Moist conditions (caused by rain, irrigation, or dew) and nighttime temperatures of 50 degrees to 60 degrees F often favor disease development. Under these conditions the fungus will produce reproductive structures that can be windblown to nearby plants or neighboring fields. Once infected, these plants will act as reservoir hosts for the secondary spread of the disease during the season. The late blight fungus may be introduced into an area by infected seed tubers, or it may survive the winter in unburied or poorly managed cull piles of infected tubers from post-harvest or preshipment grading and sorting. The fungus can also survive on unharvested volunteer plants in nearby fields.

Late blight is most often associated with destruction of tubers in storage. Tubers are often infected during digging and during harvest when fresh wounds and bruises are exposed to fungal infected vines. Tubers can also become infected when spores are washed down to exposed tuber faces at the base of the infected potato plant.

The risk of secondary infection
is particularly great on potatoes stored at temperatures of 45 degreesF. Typically, higher storage temperatures are preferred for processing quality potatoes; however, storing potatoes at these temperatures can accelerate the rate of secondary infection by other damaging organisms.

Control. Measures for controlling late blight involve both chemical control and cultural practices.

Use fungicides with azoxystrobin, chlorothalonil, cymoxanil, dimethomorph, mancozeb, or mefonoxam as the active ingredient for controlling late blight. Many of the fungicides effective in controlling early blight on potato also control late blight.

• Apply fungicides before infection for full effectiveness.
• Make applications on a weekly or biweekly schedule, beginning early in the season. Or preferably, apply fungicides before infection when weather conditions favor disease development.
• Because of the development of resistant strains of the fungus to mefonoxam fungicides, apply mefonoxam products every 14 days, and use a fungicide with a different mode of action between mefonoxam sprays.
• For successful long-term control, destroy overwintering sources of inoculum.
• Always destroy and bury infected tubers, vines, and culls to prevent existing sources of infection from moving to the new crop.
• Destroy culls by disking, burning, spreading them out to freeze, or plowing them up. Or use culls for feed, starch, or compost.