DISEASES CAUSED BY FUNGI

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1 3/20/2017 Overview Managing Fungal Diseases DISEASES CAUSED BY FUNGI Michael Harding and Ron Howard Alberta Agriculture & Forestry RJH Ag Research Potato Post-Harvest Management Workshop Lacombe, AB March 8, 2017 Black dot Black scurf Dry rot Early blight Late blight Leak Pink rot Scab Silver scurf NOVEL DISEASES OF CONCERN Zebra chip Dickeya soft rot 2 Black Dot Caused by Colletotrichum coccodes Starts with sclerotia Colletotrichum survives in the soil on crop residues and on seed tubers Black dot is often overlooked because symptoms occur late in the season and can be confused with other diseases, e.g. fusarium and verticillium wilts, rhizoctonia, early blight and silver scurf Invades underground stem tissues Airborne spores can infect the foliage Infection can progress downward New sclerotia are formed Sclerotia can overwinter and spread Tuber Infection Tubers exhibit grayish lesions on the surface that resemble silver scurf Tuber injury is more severe on thin-skinned than on netted-skinned cultivars Black Dot G. Letendre Nebraska G A Sclerotia can form on the surface of infected tubers 1

2 Control Measures Rhizoctonia Canker and Black Scurf Plant early maturing cultivars Plant when soil conditions favor rapid germination and emergence Good agronomy (avoid stress) Apply protective fungicides to the foliage Caused by Rhizoctonia solani Survives in the soil and on seed Attacks all below ground plant parts Low soil temperatures in early spring favor disease development Black Scurf Tuber Symptoms Rhizoctonia persists as mycelia and sclerotia in soil, on crop debris and on tubers Sclerotia germinate and infect emerging sprouts, roots, stolons and developing tubers Early season infection causes delayed emergence, stunting, wilting and death Low soil temperatures favor infection Control Measures Apply registered seed-piece treatments Plant when soil conditions favor rapid germination and emergence Maintain uniform soil moisture levels Fusarium Diseases There are three main types of fusarium diseases: seed piece decay, wilt and dry rot Fusarium species can attack potatoes both in the field and in storage They are able to survive harsh conditions They produce spores that can spread long distances 2

3 Many Fusarium species are involved Can be both seed- and soil-borne Tubers can become infected during cutting and handling and harvesting Seed pieces and potato roots can be directly infected from the soil Seed-piece decay and dry rot can proceed over a wide range of temps Seed-Piece Decay Seed-Piece Decay Fusarium Wilt Dry Rot Control Measures in the Field Apply registered seed-piece treatments Harvest tubers when skins are well set (top kill) Avoid damaging tubers during harvest and subsequent handling 3

4 Control Measures in Storage Grade out infected tubers Apply post-harvest fungicides Encourage suberization of tubers (55-65C) Keep storage temperatures as low as possible for the type of potato Avoid unnecessary mechanical injury during handling operations Alternaria Diseases a. Early blight (Alternaria solani) Infects potato and tomato b. Brown spot (Alternaria alternata) Also known as black pit Can be both a saprophyte and parasite on a wide variety of plant species Several conditions can predispose plants to infection: Leaf wetting Wounding and bruising Insects and nematodes Aging Poor nutrition, especially low N Other environmental stresses Early Blight Comparing Early Blight to Brown Spot Brown Spot Early Blight Brown Spot Pathogen Alternaria solani A. alternata Aggressiveness Strong/moderate Moderate/weak N deficiency Strongly favors Slightly favors Leaf damage Slightly favors Strongly favors Tuber rot Yes - lesions Yes - pits 4

5 Control Measures Scab Reduce plant stress, e.g. use high quality seed and provide optimum fertilization, irrigation and weed control Apply protective fungicides Avoid bruising tubers during harvesting Provide proper conditions for rapid suberization in storage after harvest Common scab is caused by Streptomyces scabies, an actinomycete Powdery scab is caused by Spongospora subterranea, a protist These diseases may look similar in the early stages of symptom development and may occur together on the same tuber Cause Comparing Powdery Scab to Common Scab Common Scab Powdery Scab Streptomyces scabies Spongospora subterranea Damage Tubers Roots & tubers Soil moisture Dry Moist or wet Temperatures Warm Cool Virus vector No PMTV Common Scab Streptomyces species can persist indefinitely in the soil on organic residues and plant roots They infect through lenticels on young tubers Powdery Scab Spongospora can survive for long periods in the soil as cystosori (spore balls) Spores germinate to produce zoospores which infect young tubers and roots COMMON SCAB Powdery Scab Lesions on Potato Tubers 5

6 Powdery Scab Root Galls Control Measures Common Scab Avoid planting scabby seed Plant resistant or tolerant varieties Maintain high soil moisture levels at tuber initiation Avoid using soil amendments that increase soil ph Control Measures Powdery Scab Select tolerant varieties, e.g. Russet Burbank and Ranger Russet Avoid susceptible varieties if PS potential is high, e.g. AC Glacier Chip, Dakota Pearl, Shepody, Norland, Yukon Gold Control soil moisture at tuber set Don t spread infested soil, manure or compost onto potato fields Silver Scurf Caused by Helminthosporium solani Seed- and soil-borne fungus Can persist in the soil for many years More severe on thin/white-skinned var s Symptoms increase in storage Infected potatoes are unappealing to consumers and difficult to peel for processing into French fries and chips Helminthosporium survives on tubers and in soil Silver Scurf Infects through lenticels or directly through the skin May take several months to express visible symptoms on tubers Generally worsens with time in storage 6

7 Control Measures OOMYCETE DISEASES Harvest tubers when mature with good skin set Ventilate storages to keep tuber surfaces dry Keep storage temperatures as low as possible Apply post-harvest fungicides LATE BLIGHT (Phytophthora infestans) PINK ROT (Phytophthora erythroseptica) LEAK (Pythium ultimum) DISEASE CAUSAL AGENT FOLIAGE TUBER AIRBORNE LATE BLIGHT P. infestans Yes Yes Yes PINK ROT P. erythroseptica No Yes No LEAK P. ultimum No yes No SYMPTOMS MANAGING PINK ROT & LEAK Avoid harvesting swampy areas. Allow tubers to mature, skins to set. Avoid mechanical bruising during harvest. Apply metalaxyl (Ridomil) during early bulking and/or in-furrow at planting. Apply phosphite-based fungicides at tuber initiation (then twice more two weeks apart) Harvest when temperature is below 75 F. Cure tubers for 3 weeks at F. Cool potatoes rapidly to F for storage. Keep humidity low during cooling. Force air over tubers continuously during cooling Low temperature and humidity will stop leak from growing and dry out infected tubers. MANAGING LATE BLIGHT Use clean, disease-free seed Use systemic seed treatment after cutting Eliminate cull piles, volunteers and weeds Use resistant cultivars where possible Be prepared to apply protective fungicides Use forecasting info and fungicide rotations as recommended by the PGA Destroy all foliage in hot spots Desiccate crop prior to harvest Allow tubers to mature, skins to set. Apply phosphite-based fungicides at tuber initiation (then twice more two weeks apart) Same storage recommendations as PR and Leak NEW DISEASES OF CONCERN Dickeya soft rot Zebra chip 7

8 Dickeya soft rot Dickeya spp. Can initiate disease with less inoculum Better, faster spread through vascular tissue Is more aggressive Needs a higher optimum temperature to flourish. Management of Dickeya AVOIDANCE! Use clean seed Clean and disinfect cutting and handling equip Plant seed that is well suberized Plant into well drained soil C Minimize tuber bruising and wounding Store tubers with good air flow, humidity and as cool as possible. Zebra chip "Candidatus Liberibacter solanacearum, Vectored by the potato psyllid Causes dark stripes in tuber flesh and potato chips Management of Zebra chip Scout for and test psyllids for Lso Remove and destroy infected plants QUESTIONS? 8