b'ularlyshapedandoftenwithouta defined outline. The black dots (microsclerotia), which differentiatethisdiseasefromsilver scurf, are easily seen under a hand lens (10x magnification). In very susceptible varieties, the lesions are black and cover largerareasofthetuber.Eyeswithin Blacklegofpotatoseedtubers.Fusarium seed piece decay in tubers.affectedareasproduceweaksprouts (EugeniaBanks,OntarioPotato(EugeniaBanks,OntarioPotato Board)Board)resultinginpoorstandsifseverely infected seed is planted. Dickeya (Dickeya dianthicola) The symp- LateBlightSeed(Phytophthorainfes-toms of Dickeya tuber rot are very simi- tans)lar to blackleg, however Dickeya tuberExternaltuberlesionsareslightly rot often looks cheesy, and it is usual- sunken, brown to purplish areas of vari-lynotassmellyasblackleg.Healthy- able size. The internal symptom is a tan lookingseedtubersmaycarrytheto brown, granular dry rot. There is no Dickeya bacterium in a dormant state;clearseparationbetweeninfectedand such tubers usually rot and plants wilthealthy tissue. Often, soft rot bacteria when spring temperatures are close toinvadeinfectedtubers.Infectedseed 77oF (25oC.)may rot in the field or weak, sick plantsBlackdotinfectiononseedtubers. may emerge. Later, lesions that develop(EugeniaBanks,OntarioPotato ontheseplantsproducethousandsofBoard) sporesthatcanstartanepidemicifRhizoctoniaBrownCankersOn weather is cool and wet.Sprouts (Rhizoctonia solani)A symptom of this disease is the forma-tionofbrowncankersonsprouts. Brown cankers may pinch off sprouts, resulting in poor emergence.Dickeyainseedtuber.(Eugenia Banks, Ontario Potato Board)Fusarium Seed Piece Decay (Fusarium spp.)Tuber infection occurs at wound sites. ExternaltuberlesionsaresunkenandLateblightseedinfection(photo shriveledwithconcentricwrinkles.credit Eugenia Banks).Rotting flesh may be brown or grey to black,dry,andcrumbly.CracksoftenEye Or Sprout InfectionsRhizoctoniaonsprouts.(Steve Johnson,UniversityofMaine forminthelesionsandcavitiesareCooperative Extension) often lined with white, pink, yellow, orBlack Dot ( Colletotrichum coccodes) Physiological Conditionsblue mold develop on the rotted tissue.This fungus produces light brown blem-Infected seed pieces may be partially orishes with microsclerotia on the tuberChilling injury completely destroyed in the soil result- skin. Later, unsightly dark brown patch-ing in uneven stands.es can develop. Lesions tend to be irreg- Thiscanoccurbytuberexposureto MAY/JUNE 2023 POTATO GROWER 11'