Black leg and light leaf spot outbreaks in Oregon Cynthia Ocamb, PhD. Extension Plant Pathologist Oregon State University Botany & Plant Pathology ocambc@science.oregonstate.edu 541-737-4020
Fungi that cause black leg and light leaf spot: produce ascospores on infected plant residues atop the soil surface; these spores can be wind-blown for miles produce asexual spores (conidia) that develop on leaf spots, stem cankers and crop residues; spread relatively short distances by rain both diseases promoted by cool, moist conditions (46-60 F) pathogens are seedborne Black leg Light leaf spot
Black leg & light leaf spot outbreaks began in 2014 Fungi cause black leg [Leptosphaeria maculans (Phoma lingam) & Leptosphaeria biglobosa] and light leaf spot [Pyrenopeziza brassicae (Cylindrosporium concentricum)] seedborne and survive in infected plant residues atop soil Hosts include: Brassica - broccoli, cabbage, canola, kale, rutabaga, turnip Raphanus - daikon, radish Sinapis - white and yellow mustard weeds - tansymustard, wild mustards (birdsrape), wild radish, wild cresses (pennycress, western yellowcress, etc.)
Black leg-infected seedlings may die, resembles damping-off Phoma growing inside a stem of mustard greens leaf spots on radish due to seedborne Phoma
Plants that survive the seedling stage are stunted 37-acre conventional processing broccoli field planted with infected seed - averaged 2 tons/acre, well below the processor s estimate of 5.5 tons/acre
Lesions develop on the stem near the soil surface, eventually girdling the plant s main stem.
Mean # of leaf spots (weekly average of 16 plants) 300 250 200 150 100 50 Black leg on trap plants in Oregon 0 Sep Nov Jan Mar May 2015 2016 Mean # Phoma leaf spots/plant Trap plants 2016-2017 crop cycle 10/1/16 11/1/16 12/1/16 1/1/17 No. of leaf lesions reflects the intensity of windblown ascospores. First infections of trap plants occurred mid-oct in 2015 & 2016. 120 100 80 60 40 20 0 Turnip 2016 Canola 2016 Canola 2014 Turnip 2016 Turnip 2016
Pseudothecia produced on infected crop residues, release ascospores Asci with ascospores Pycnidia oozing conidia Ascospores are moved by wind at least several miles
Pycnidia oozing conidia (moved by splashing water) Organic broccoli leaf Turnip residue
Black leg on fresh-market rutabaga
Black leg on fresh-market kale
Black leg on fresh-market broccoli
Light leaf spot appeared in North America (Oregon) in 2014 causes leaf spots, leaf deformations, defoliation and plant stunting
Light leaf spot remains asymptomatic until late winter/early spring (after infection occurs during late fall) LLS-infected plants are more sensitive to freeze injury LLS is difficult to diagnosis
Light leaf spot on Brassica leaves
Light leaf spot on plant stems
Light leaf spot on stem compared to black leg
Light leaf spot and black leg on fresh-market turnip
Disease management in organic Brassica crops for black leg and light leaf spot Rotate out of crucifers for at least three years. Avoid planting within ¼ mile of a previously-infected field. Do not plant crucifer species as cover crops; use cereals, vetches, and/or clover, etc. Plant spring or summer crucifer crops (disease pressure high in overwintered crops). If possible, protect fall-planted crucifers from rain.
Scout Brassica crops for black leg and light leaf spot. Remove infected leaves or rogue symptomatic plants. Black leg Light leaf spot
If plantings are infected with black leg or light leaf spot: Do not leave vegetable crop plants standing in the field after harvest. Incorporate or remove infected residues after harvest bury residues, hand-removal, etc. Residues atop soil surface are infectious until decomposed!
Control crucifer volunteers and susceptible weeds
Hot Water Seed Treatment Steps for hot water treatment Pre-warm seed in water Treat seed with hot water Cool seed Dry seed 20-30 minutes at 122 F recommended for Brassica seeds Ohio State University extension fact sheet on hot water seed treatment (HYG-3086-05) by Sally Miller and Melanie L. Lewis Ivey
Treatment time for Crucifer crops Type of seed Water temperature Minutes Radish, mustard, cress 122 F 15 Broccoli, cauliflower, collard, 122 F 20 kale, kohlrabi, rutabaga, turnip Brussels sprouts, cabbage 122 F 25
Tools Needed 2 hot water baths or double boilers 2 thermometers Container to cool seed Container to hold seeds Screens or tarps/sunshine for drying seed
Using fiberglass screen for heat treating seed. 1. Cut screen piece large enough so that the seed lot fits loosely inside. 2. Fold once, fold and staple two sides, leaving the third open so that seed may be inserted. 3. Add seed, fold and staple final side. Add a marker with seed name or code (plant tag with permanent marker shown here).
Using #4 cone-style coffee filters for heat treating seed 1. One cone-style filter and a weight (quarter shown here) to prevent floating. 2. Place seed inside, no more than half full. Include weight. 3. Fold and staple top. Add a marker with seed name or code (plant tag with permanent marker shown here).
It is important to treat small amounts of seed at a time The seed needs to heat quickly and then cool quickly. Agarwal, V. K., & Sinclair, J. B. (1996). Principles of seed pathology. Crc Press; Boca Raton, Florida
Prewarm seeds for 10 minutes at 100 F
Soak seed for 15 to 30 minutes in hot water maintained at 122 F Cool seed in (running) tap or well water for 5 minutes
Seed drying Seeds can be dried on screens or spread ¼-inch layers on reflective tarps in direct sunlight. Smaller amounts of seed can be dried overnight in single layers on top of absorbent paper (news papers, paper towels, etc.) with or without a fan to circulate air. Many seeds cannot tolerate drying temperatures > 85 F.
Clean equipment between lots of seed 1) Thoroughly wash with hot, soapy water or 95% alcohol Thermometers Containers for heating water Seed drying equipment 2) Sterilize with boiling water Tea balls, cheesecloth, wire-screens Containers for heating water Seed drying equipment
Hot Water Seed Treatment is an Art Form The tools used to heat the water and contain the seed can vary depending on your situation. The margin of error is very small when treating seed: Treat too long and the seed will be killed Treat less than the optimum and the pathogen will not be killed
Hot water treatments will aid in general control of seedborne pathogens on crucifers, carrot, celery, eggplant, lettuce, onion, parsnip, pepper, spinach, tomato Phoma species (black leg) on crucifers other pathogenic fungi (Alternaria, Fusarium, Verticillium, etc.) bacteria (Xanthomonas, Pseudomonas, etc.) fungicide seed treatments were developed in response to seedborne pathogens
Control: 92% seed has Phoma Same seed with 20-minute hot water treatment 11.5 % seed with Phoma
Cornell Temperature and Time Recommendations http://vegetablemdonline.ppath.cornell.edu/newsarticles/hotwater%20seed%20trt%20protocols.pdf
LSU Extension Hot Water Treatment Video https://www.youtube.com/watch?v=ie3h_ickwb4 Here is an 8 minute video available free on Youtube, watch when you need to refresh your memory!
For more information on diseases PNW Plant Disease Management Handbook http://pnwhandbooks.org/plantdisease/ Hot water seed treatments Google search Ohio State + hot water seed treatment www.oardc.ohio-state.edu/sallymiller/extension/factsheets/organicseedtrt.pdf