Late blight – Phytophthora infestans, fungus
Tubers can be infected during the growing season by spores washed from foliar lesions or at harvest if tubers are dug before infected vines are thoroughly killed or while vines are still wet. Infected tubers may feel very firm to the touch. External tuber symptoms consist of patches of brown to purple discoloration on the skin, which become darker and sunken with time. When tubers are cut through these lesions, a reddish brown (mahogany), dry, firm rot is visible, progressing up to '/z inch deep into the cortex. These lesions have a somewhat granular appearance and spread unevenly into the tuber, particularly if the tubers have been stored for some time. Isolation of the causal agent may be necessary for an accurate diagnosis. This disease may be confused with pink eye, a disease not discussed here. Healthy tubers can be infected in storage if temperature and moisture are favorable. Invasion by soft rot bacteria often results in a wet rot. Tubers with evidence of late blight should be discarded before storing, and storages should be maintained at 36°–40° F with forced–air ventilation. Control in the field relies upon clean tubers, proper hilling, foliar fungicide applications, and proper vine–killing procedures. When the fungus is present on the foliage, delay harvesting until vines have been dead for a minimum of 2 weeks. Cull piles and volunteer plants are important sources of inoculum and should be disposed of.