M A Y / J U N E 2 0 1 7 22 POTATO GROWER However, the margin between late blight infected tissue and healthy tissue is frequently marked by brown fingerlike extension into the healthy flesh of the tuber. Infections of other tuber rot organisms such as soft rot bacteria, pink rot, or leak may frequently use late blight affected areas as infection courts and frequently progress more rapidly than P. infes- tans making diagnosis difficult. Positive identification of late blight can be made by microscopic examination of sporulating sam- ples from infected leaves or tubers. Identification of non-sporulating samples and identification of late blight genotypes can be done by polymerase chain reaction, com- monly referred to as PCR. This service is available free of charge at NDSU in Dr. Neil Gudmestad’s lab- oratory. Some late blight geno- types have been documented to have resistance to the fungicide mefenoxam/metalaxyl. When late blight is confirmed, it is essential to determine the late blight geno- type to ensure effective chemical treatments. Contact the local Extension service for current information on fungicide resistant late blight genotypes. Disease Cycle P. infestans, the cause of late blight, is a heterothallic fungal- like pathogen, meaning there are two mating types required for sex- ual reproduction and are referred to as A1 and A2. The pathogen is an obligate para- site, that cannot survive without a living host. However, in some areas of the world where both mating types of the pathogen are pres- ent a sexual spore is produced that is capable of soil sur- vival. Although soil survival is not known to defini- tively occur in the USA, there is anec- dotal evidence that sexual combina- tion has occurred suggesting soil sur- vival is possible. Infected potato tubers are the pri- mary source of inoculum for P. infestans, includ- ing potatoes in storage, infected tubers missed dur- ing harvest that remain not frozen during the winter (volunteers), seed tubers, infected cull piles or on other host plants. The pathogen can be transmitted from infected seed tubers to newly emerging potato Figure 3. Leaf lesions are also frequently surrounded by a yellow chlorotic halo. (Photo credit: Andy Robinson, NDSU/UMN) Figure 4. Late blight lesions may be present on petioles and stems of the plant, especially in new growth where moisture persists. (Photo credit: Andy Robinson, NDSU/UMN) Figure 5. During active late blight growth, especially in cool wet weather, a white mildew-appearing area is vis- ible at the edge of the lesions. (Photo credit: Eugenia Banks, Ontario Ministry of Agriculture and Food)