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Sources, spread and survival of pathogens, pests and weeds in ornamental crops
Learn about the sources, spread and survival of common pathogen, pest and weed problems by cropping area and how to control them.
Back to: Guidelines on nursery hygiene for outdoor and protected ornamental crops
A checklist of the common ways pests, pathogens and weeds arrive and spread on a nursery, arranged by inspection area, is provided in Table 1.
Many pest and disease organisms have more than one method of spread, such as via air currents, on crop debris or by direct contact. This makes them difficult to control, but attempts should still be made. Preventing or delaying introductions of pests, pathogens, and weeds onto crops reduces the risk of damaging populations building up.
Information on the survival of selected pests, pathogens and weeds is summarised in Table 2. Many can survive for months or even years in the absence of a crop.
Once used, containers and trays need to be cleaned and treated with disinfectant before reuse to kill pathogens, pests and weed seeds, and covered after treatment
Image © ADAS Horticulture
Dirty floor or bench coverings can be a source of root-infecting pathogens, especially Fusarium, Pythium, Phytophthora, Rhizoctonia and Thielaviopsis, and some pests
Image © AHDB
Table 1. A checklist of the common pathogens, pests and weeds in ornamental crop production, arranged by nursery area/input, with outline control measures
Nursery |
Type of problem spread this way |
Control measures |
Glasshouse structure |
Viruses, some fungal pathogens, overwintering pests, e.g. two-spotted spider mite |
Clean and disinfect. Control spider mites on the crop before they enter diapause and overwinter |
Benches and matting |
Some pathogens (especially root-infecting water moulds/oomycetes such as Pythium and Fusarium). Some pests with ground-dwelling life stages, e.g. thrips, sciarid and shore flies, leaf miner |
Clean and disinfect. Renew old matting. Remove dead or dying plants and debris. If appropriate, apply biological control agents against insects and mites, e.g. ground-dwelling predatory mites, beetles or nematodes and bioprotectants for disease control |
Floors |
Some pathogens in debris and dust; various pests (as for benches and matting), also slugs, snails, woodlice and vine weevils; insects and nematodes on plant debris or feeding on algal growths; moss and liverwort |
Clean the floor, control algal growth, mosses, and liverwort. Renew covering. Level hollows to eliminate puddles. Apply biological control agents and bioprotectants if appropriate, as for benches and matting |
Machinery and equipment |
Fungal and oomycete spores, bacteria, viruses, weed seeds, pests, e.g. thrips and leaf miner pupae |
Regularly clean seeders, transplanters (especially fingers on machines), potting machines, secateurs, knives etc. Keep work areas clean |
On wheels of fork lift trucks, soles of shoes, etc. |
Oomycetes, root-infecting pathogens, viruses, pests with a life cycle stage on the ground e.g. thrips and leaf miner pupae |
Allocate specific vehicles to high-risk areas. Clean vehicles regularly. Place disinfectant-soaked matting across entrances to high-health status areas |
Cropping aspects |
Type of problem spread this way |
Control measures |
Seed |
Some fungal and bacterial pathogens, less commonly viruses; contaminating weed seeds |
Use certified, high-quality seed. Request seed treatment where there is a known risk of a seedborne pathogen. Seed testing |
Incoming cuttings and young plants |
Some fungal, bacterial and viruses; some pests, e.g. thrips, leaf miner, whitefly, nematodes and vine weevil larvae (in plugs or liners) |
Inspect and quarantine incoming stock; use sticky traps. Use virus-tested stock. Use stock indexed for vascular pathogens. Avoid placing plants on the ground or a dirty work surface. Apply biological control agents or bioprotectants if required |
New containers |
Pathogens, weed seed |
Regularly brush or vacuum pathways and areas around potting machines. Store under cover or shrink-wrapped if outside. Store old pots separately from new ones |
Reused containers |
Some pathogens, particularly their resting stages (e.g. Thielaviopsis, cause of pansy black root rot); weeds (e.g. bittercress around edges of old containers); slugs and snails under trays, pests on plant debris, e.g. thrips, leaf miner, nematodes, spider mite |
Clean and disinfect before reuse (e.g. high-pressure wash the trays using water containing a disinfectant). Note: disinfectants alone may not kill weed seeds; cleaning first is essential |
Growing media, soil and dust |
Occasional pathogens, weed seed (especially in dust), some pests |
Protect growing media from dust and run-off water. Cover bunkers and part-used or damaged bags. Agree growing media specification and keep samples |
Water |
Some fungal pathogens (especially oomycetes), bacteria and weed seed |
Cover storage tanks. Routinely test recycled and stored water, and storage tanks, for plant pathogens. Treat recycled water. Check water treatment equipment is working correctly. Clean and disinfect irrigation lines. Avoid irrigating with contaminated water, especially young plants. Do not drop hoses on dirty ground. Avoid water splash. Keep reservoir banks free of weeds and plant debris |
Crop covers (e.g. fleece) |
Some pathogens and pests, in particular resting spores of pathogens or pupae |
Dispose of contaminated crop covers and only reuse crop covers in low-risk situations |
Growing and standing areas |
Weeds, pathogens, pests. On sloping sites, run-off water can spread root pathogens to plants lower down the slope (especially Phytophthora) |
Sweep floors and benches regularly, disinfect as necessary. Ensure standing areas are adequately drained. Ensure ground cover material is in good repair and covers all the growing area up to path edges. Replace capillary matting as necessary. Skim sand beds. Check uniformity and distribution of irrigation. Remove diseased and infested plants promptly. Put IPM strategies in place on new crops promptly. Seek to devise production routes to minimise old crops contaminating new crops. Do not set down new crops in spaces between old crops |
Insect movement |
Flying pests, e.g. aphid, adult thrips, leaf miner, leafhopper, whitefly, sciarid and shore flies. Crawling pests, e.g. slugs, snails, adult vine weevil. Some fungi and viruses carried within insects (e.g. Pythium and Fusarium with sciarid flies, tomato spotted wilt virus with western flower thrips) |
Use sticky traps to monitor insect levels. If appropriate, use roller traps for mass trapping of pests. Monitor indicator plants. Control pests that can cause crop damage or transmit disease |
Air currents |
Small pests, e.g. thrips, some fungal pathogens, wind-blown weed seed |
Use sticky traps to monitor for thrips and other pests. Consider screening vents with insect-proof mesh, or use sticky traps under vents or next to doors. Use windbreaks to reduce weed seed blown in from surrounding fields |
Contact between plants |
Botrytis, crawling pests, e.g. spider mite, aphid, thrips larvae |
Remove affected plants. Give adequate plant spacing (unless using biological control agents, e.g. Amblyseius species and Phytoseiulus persimilis, which need plants to be touching) |
Waste plants and crop debris |
Some fungal pathogens and pests, e.g. western flower thrips (WFT) on unmarketable flowering plants |
Place unmarketable plants and debris in covered skips or other covered containers. Cover any heaps of plant waste near the nursery. If composting in open heaps, locate them at least 10 m away from production areas |
Non-cropping aspects |
Type of problem spread this way |
Control measures |
Non-cropped areas |
Pests on weeds, e.g. whitefly and leaf miner on chickweed and sowthistle under benches or around edges of glasshouse. Occasional fungal pathogens on weeds (e.g. rusts, botrytis). Weed seeds |
Control weeds in and around cropping areas and outside glasshouses, especially next to stored growing media, water tanks and reservoirs. Use vegetation-free strips or closely mown grass around houses and production areas. Do not allow rubbish to accumulate near growing media mixing areas |
Dispatch areas |
Pathogens, pests and weed seed on trolley shelves and wheels of trolleys, forklift trucks and lorries |
Clean floor regularly. Clean and disinfect trolleys before reuse. Devise routes to avoid cross-contamination between dispatch and production areas |
Staff and visitors |
Some fungal and viral pathogens and pests, e.g. powdery mildew, spider mite and thrips on clothing |
Ensure staff are trained in and follow nursery hygiene procedures; display the nursery hygiene rules and practices as a reminder. Ensure visitors and contractors working on-site report to reception and follow the nursery hygiene policy. Avoid staff movement from infected/infested areas to ‘clean’ areas. Use protective clothing, gloves and hand-sanitisers in high health status areas. Use disinfectant foot dips. Wash hands regularly. Do not allow staff to raise their own plants on site unless a risk assessment indicates minimal risk |
Table 2. Persistence of some key pathogens, pests and weeds on structures or in the soil or growing media
Pathogen |
Persistence |
Comment |
Black root rot dispersal spores |
4–5 years in root tissue |
May occur in nursery sweepings. Capable of prolonged saprophytic survival in soil |
Botrytis cinerea dispersal spores |
Weeks to around 1 year |
Many environmental factors reduce survival (e.g. sunlight, frequent wetting and drying). Ability to infect plants may decline more rapidly than ability to germinate |
Downy mildew dispersal spores |
Days |
Information relates to Peronospora parasitica on leaves under typical field conditions. Spores desiccate. Longer-lived resting spores can be formed |
Fusarium oxysporum resting spores |
Years |
Cyclamen fusarium wilt tends to reoccur in a nursery after a severe outbreak, possibly due to persistence between crops on matting or benches. Long-lived in soil |
Powdery mildew dispersal spores |
Days |
Washing a glasshouse with water can trigger spore germination and reduce survival |
Pythium resting spores |
Years |
Will survive drying on a range of surfaces |
Phytophthora dispersal spores |
Weeks |
Varies with species. Some species, e.g. P. ramorum can be dispersed into the air, others, e.g. P. cactorum, are shed into water |
Phytophthora resting spores |
Months to years |
Varies with species, but generally several years |
Rhizoctonia solani resting structures |
Years |
Capable of prolonged saprophytic survival in soil |
Sclerotinia sclerotiorum resting structures |
6–8 years |
Contans WG is available for biological control of soilborne resting structures; also consider soil disinfestation or growing out of the soil if a serious problem |
Soft rot bacteria, e.g. Pectobacterium carotovorum |
Days |
Commonly occurs in soil and water, so re-infestation may readily occur |
Tomato mosaic virus (ToMV) |
Years |
Can survive for years in plant debris |
Pest |
Persistence |
Comment |
Aphid |
Weeks to months |
Most species do not produce eggs. Aphids will only survive on host plant material, e.g. crop plants, weeds or leaf debris |
Caterpillar and moth pupae |
Months |
Some species pupate in soil or growing media and can overwinter. Other species, e.g. carnation tortrix can overwinter as larvae in rolled up leaves of host plants |
Glasshouse whitefly |
Weeks to months |
Will only survive on host plants, weeds or leaf debris. Can overwinter in glasshouses and on weeds outdoors in mild winters |
Leaf miner pupae |
Weeks to months |
Liriomyza species can survive for several months in soil or growing media, depending on species and temperature. Native Chromatomyia species pupate in the leaf and can survive in leaf debris |
Two-spotted spider mite |
Months |
Can overwinter in glasshouse structure and equipment |
Vine weevil |
Months |
Larvae and pupae (both in soil or growing media) and adults (in sheltered refuges) can be found all year round in glasshouses but usually overwinter as larvae and pupate and emerge as adults in spring |
Western flower thrips (WFT) |
Months |
Depending on temperature and moisture, may overwinter as adults, larvae or pupae, on plants, plant debris, weeds or in soil |
Weeds |
Persistence |
Comment |
Annual meadow grass |
> 5 years |
|
Chickweed |
Many years |
Most germinate within a year, but a small percentage can remain dormant for 60+ years |
Common sorrel |
> 5 years |
Occasionally problematic in growing media |
Groundsel |
> 5 years |
Most germinate within a year, but a few can remain viable for longer |
Hairy bittercress |
> 5 years |
|
Liverwort spores |
Around 1 year |
Spores kept at room temperature were 100% viable after a year, 50% after 14 months and failed to germinate totally after 17 months |
Useful links
Read the AHDB crop walker guides for bedding and pot plants, cut flowers and hardy nursery stock to familiarise yourself with the range of issues encountered in the production of ornamentals.
Crop Walkers' Guide: Bedding and pot plants
Crop Walkers' Guide: Cut flowers
Crop Walkers' Guide: Hardy Nursery Stock
Authors
The content for these web pages were originally authored for AHDB by:
Author(s) – Dave Kaye and Erika Wedgwood. ADAS Horticulture.
Original author(s) – Tim O’Neill, Wayne Brough, John Atwood and Jude Bennison, ADAS.
Webpage content correct as of June 2021.
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