b'ularlyshapedandoftenwithoutadefined outline. The black dots (microsclerotia), whichdifferentiatethisdiseasefromsilverscurf, are easily seen under a hand lens(10x magnification). In very susceptiblevarieties, the lesions are black and coverlargerareasofthetuber.EyeswithinBlacklegofpotatoseedtubers. Fusarium seed piece decay in tubers. affectedareasproduceweaksprouts(EugeniaBanks,OntarioPotato (EugeniaBanks,OntarioPotatoBoard)Board) resultinginpoorstandsifseverelyinfected seed is planted.Dickeya (Dickeya dianthicola) The symp- LateBlightSeed(Phytophthorainfes-toms of Dickeya tuber rot are very simi- tans) lar to blackleg, however Dickeya tuber Externaltuberlesionsareslightlyrot often looks cheesy, and it is usual- sunken, brown to purplish areas of vari-lynotassmellyasblackleg.Healthy- able size. The internal symptom is a tanlookingseedtubersmaycarrythe to brown, granular dry rot. There is noDickeya bacterium in a dormant state; clearseparationbetweeninfectedandsuch tubers usually rot and plants wilt healthy tissue. Often, soft rot bacteriawhen spring temperatures are close to invadeinfectedtubers.Infectedseed77o F (25o C.) may rot in the field or weak, sick plants Blackdotinfectiononseedtubers.may emerge. Later, lesions that develop (EugeniaBanks,OntarioPotatoontheseplantsproducethousandsof Board)sporesthatcanstartanepidemicif RhizoctoniaBrownCankersOnweather 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.(EugeniaBanks, Ontario Potato Board)Fusarium Seed Piece Decay (Fusariumspp.) Tuber infection occurs at wound sites.Externaltuberlesionsaresunkenand Lateblightseedinfection(photoshriveledwithconcentricwrinkles. credit Eugenia Banks). Rotting flesh may be brown or grey toblack,dry,andcrumbly.Cracksoften Eye Or Sprout Infections Rhizoctoniaonsprouts.(SteveJohnson,UniversityofMaineforminthelesionsandcavitiesare Cooperative Extension)often lined with white, pink, yellow, or Black Dot ( Colletotrichum coccodes)Physiological Conditions blue mold develop on the rotted tissue. This fungus produces light brown blem-Infected seed pieces may be partially or ishes with microsclerotia on the tubercompletely destroyed in the soil result- skin. Later, unsightly dark brown patch- Chilling injurying in uneven stands. es can develop. Lesions tend to be irreg- ThiscanoccurbytuberexposuretoMAY/JUNE 2021 POTATO GROWER 19'