Chile Pepper vs Soilborne Pathogens: Can the plant win? Michael Matheron Extension Plant Pathologist & Professor Yuma Agricultural Center matheron@ag.arizona.edu

Known diseases of pepper Cause Number Bacteria 4 Fungi and Oomycetes 14 Viruses 17 Nematodes 10 Parasitic seed plant 1 Postharvest TOTAL 50 From Compendium of Pepper Diseases

Phytophthora blight Verticillium wilt Rhizoctonia root rot

Phytophthora blight Verticillium wilt Rhizoctonia root rot All caused by soilborne pathogens Adapted to live and survive in soil Invisible to the naked eye

Questions related to disease What is the problem ? What is the cure ?

Questions related to plant diseases What is the problem ? What can I do to prevent or manage the problem ?

To effectively manage a plant disease, we need to know what affects its development

Requirements for disease development severity Susceptible Host plant Favorable environment Virulent pathogen

Phytophthora blight of pepper. Pathogen: Phytophthora capsici Phytophthora blight of pepper Pathogen: Phytophthora capsici First described in 1922 in New Mexico Phytophthora means ‘plant destroyer’ (Greek)

Phytophthora blight of pepper Symptoms Images courtesy of D. Lindsey

Phytophthora blight of pepper Symptoms

Oomycetes initially described as fungi, but over time, significant differences compared to true fungi became apparent Characteristic Oomycetes True fungi Primary cell wall component Cellulose Chitin Vegetative nuclear state Diploid Haploid Asexual spores with flagella Yes No Several biochemical differences as well, such as Reaction to specific fungicides

Disease cycle for Phytophthora blight Ristaino & Johnston. Plant Disease 83:1080-1089

Management considerations for Phytophthora blight of pepper The host Host resistance or tolerance to Phytophthora capsici is a desired goal

Management considerations for Phytophthora blight of pepper The pathogen Inoculum avoidance Use plants and transplants free of the pathogen Inoculum survival Crop rotation to nonhost plants Solarization of soil Some organic amendments can affect survival of Phytophthora in soil

Management considerations for Phytophthora blight of pepper The pathogen Deployment of fungicides

Phytophthora disease management Protectant fungicides Introduction date Copper sulfate + lime (Bordeaux mixture) 1885 Cuprous oxide and other copper salts 1932 Dithiocarbamates (Zineb, Maneb, Mancozeb, 1951-1962) 1931-1962 Phthalimides (captan, captafol, folpet) 1949-1965 Triphenyl tin compounds (fentin acetate or fentin hydroxide) 1954 Chlorothalonil 1963

Phytophthora disease management Systemic fungicides Introduction date Isoxazoles (hymexazol) 1974 Cyanoacetamide Oximes (cymoxanil, Curzate) 1976 Phenylamides (metalaxyl, mefenoxam, Ridomil) 1977 Phosphonates (fosetyl-Al, Aliette; phosphorous acid salts) Carbamates (propamocarb, Previcur) 1978 Dimethomorph (Acrobat, Forum) 1988 Dinitroanilines (fluazinam, Omega) 1992 Strobilurines (azoxystrobin, kresoxim-methyl) Zoxamide 2004

Fungicides active against Phytophthora Target site Common name Trade name FRAC Code Nucleic acid synthesis Mefenoxam Ridomil Gold 4 Mitosis & cell division Zoxamide Gavel 22 Ethaboxam Fluopicolide Presidio 43 Respiration Strobilurines Cabrio, Flint, Quadris 11 Famoxadone Component of Tanos Fenamidone Reason Cyazofamid Ranman 21 Fluazinam Omega 29 Ametoctradin Component of Zampro 45 Cell wall biosynthesis Dimethomorph Forum 40 Mandipropamid Revus

Fungicides active against Phytophthora Target site Common name Trade name FRAC Code Lipid synthesis and membrane integrity Propamocarb Previcur 28 Unknown mode of action Cymoxanil Curzate 27 Fosetyl-Al Aliette 33 Phosphorus acid & salts Phostrol, etc. Oxathiapiprolin Orondis U 15 Multisite contact activity Copper salts Kocide, etc. M 1 Mancozeb Manzate, Dithane M 3 Chlorothalonil Bravo M 5

Management considerations for Phytophthora blight of pepper The environment Soil and atmospheric water Since Phytophthora is a “water mold”, water management is critical Proper placement and duration of irrigation Enhance movement of water through soil Provide adequate field drainage in areas subject to high levels of rainfall

Management considerations for Phytophthora blight of pepper The environment Predisposition of plants to disease Drought stress Waterlogged soil Soil salinity

Verticillium wilt of pepper. Pathogens: Verticillium dahliae Verticillium wilt of pepper Pathogens: Verticillium dahliae Verticillium albo-atrum Verticil: A circular arrangement, as of flowers or leaves, about a point on an axis. A whorl.

Verticillium wilt of pepper. Pathogens: Verticillium dahliae Verticillium wilt of pepper Pathogens: Verticillium dahliae Verticillium albo-atrum Verticil: A circular arrangement, as of flowers or leaves, about a point on an axis. A whorl. Courtesy of Berlanger and Powelson

Verticillium wilt of pepper: Symptoms Images courtesy of D. Lindsey

Disease cycle for Verticillium wilt Courtesy of Berlanger and Powelson

Verticillium microsclerotia forming on dying plant tissue Courtesy of Berlanger and Powelson

Verticillium wilt of pepper Preplant management options Plant resistant or tolerant varieties if available Crop rotation to nonhost plants Verticillium has large host range (> 200 plant species) Microsclerotia can persist in soil for several years Certain crops (broccoli) can significantly reduce microsclerotia population in soil Chemical soil fumigation Effective but expensive Application restrictions, loss of some products Soil solarization

Verticillium wilt of pepper Management options in growing season Apply optimal rates of nitrogen & phosphorus Effective on some plant species Avoid overwatering On young plants of some species, overwatering increases infection and subsequent disease severity

Verticillium wilt of pepper Management after harvest Propane flaming Destroy Verticillium microsclerotia in crop residue Peppermint field

Rhizoctonia root rot of pepper. Pathogen: Rhizoctonia solani Rhizoctonia root rot of pepper Pathogen: Rhizoctonia solani Origin of the name ‘Rhizoctonia” rhiza (Greek for ‘root’) ktonos (Greek for ‘murder’)

Rhizoctonia root rot of pepper: Symptoms Courtesy of G. Holmes Courtesy of P. Bosland

Rhizoctonia solani

Disease cycle for Rhizoctonia diseases From Agrios, et al.

Rhizoctonia root rot of pepper Management options Plant resistant or tolerant varieties if available Use fungicide treated seed Rhizoctonia solani has a very large host range Avoid saturated soil Avoid planting in soil having large amounts of undecomposed plant residue Could enhance growth and survival of Rhizoctonia solani in soils

Chile Pepper vs Soilborne Pathogens: Can the plant win? There is no silver bullet Develop and deploy an Integrated Disease Management Plan incorporating all tools that will contribute to reducing disease development to the maximum possible extent