Pests and Pathogens in Propagation Facilities

1. Management program begins prior to propagation with preventive measures, identifying and eliminating the possibility of contamination

a) The propagation facilities: Greenhouse structures, greenhouse floors, pots, flats, hand tools, hoses, benches, etc. can all harbor plant pathogens. Good sanitation programs should include periodic cleaning or disinfecting of all materials and facilities.

b) Propagation media can be another source of contamination, especially for soil borne bacteria/fungi and weed seeds (see Lecture 4, Soil Media, Fertility, and Container Formats). To minimize this risk, growers can:

i. Use biologically active, disease-suppressing media based on high quality composts, and/or inoculated with beneficial fungi or mycorrhizae 

ii. Use sterile, soilless media that comes from sterile sources, lacks biological potential, or has been previously treated to eliminate pathogens

iii. Use heat/steam and solar pasteurization methods to sterilize media, a costly but effective method that will eliminate pathogens and beneficial organisms simultaneously

c) Seed/plant stock can also be a source of contamination. The grower can protect against this potential by:

i. Using seed/propagule material that comes from reliable sources and is certified to be pest and disease free

ii. Using seed pre-treatment techniques such as hot water baths to kill fungi and other pathogens

d) Exclude pests from growing environment

i. Screen at all points of entry into the greenhouse, including vents, fans, and doorways

ii. Use floating row covers over cell trays to keep flying insect pests off of emerging crops

iii. Use physical barriers such as water basins or sticky resins on table legs to prevent ants and other crawling insects from having access to young crops

2. Good cultural practices are a critical component in the management/prevention of pest/ disease challenges

a) Select pest- and disease-resistant varieties and avoid crops vulnerable to known potential problems. Check with local growers and extension agents for issues common in your area for the crops you grow.

b) Grow crops at appropriate seasonal junctures, where environmental conditions naturally facilitate healthy, vigorous, pest- and disease-resistant growth

c) Manage environmental conditions to mitigate against the presence of pests/disease and promote vigorous, uninterrupted growth. This includes the management of:

i. Temperature: Especially important in the prevention of damping off organisms, which thrive when soils are constantly moist and temperatures are steadily in the 68ºF to 86ºF range. While this range is both ideal for damping off organisms and for the growth of many common crops, damping off damage can be prevented by using high quality soil media, making sure the soil goes through adequate wet to dry swings, and sacrificing optimal temperatures when cooling will control damping off fungi. 

ii. Moisture: The quantity and frequency of moisture delivery is critical to healthy seedling development. Constantly wet soil deprives roots of necessary oxygen, limits the mobilization of organic nutrients in the soil mix, and can create conditions that favor damping off and root rotting fungi. Excess irrigation can also lead to nutrient leaching from the soil media, depriving plants of valuable resources and potentially compromising local surface or groundwater quality (see Supplement 2, Conserving Water and Protecting Water Quality).

iii. Air circulation: Circulation or oxygen exchange within the greenhouse, as previously highlighted, helps regulate greenhouse temperatures, is critical in promoting strong cells and healthy growth, and prevents pathogen buildup

iv. Fertility: In concert with other cultural practices, adequate but not excessive soil fertility promotes healthy, uninterrupted growth. Excess fertility can lead to lush, rangy growth and attract aphids and other insects that feed on nitrogen rich crops (see Appendix 6, Sample Soil Mix Recipes, for examples of mixes with appropriate fertility).

3. Management also includes monitoring and early detection of pest/disease problems to minimize crop loss and need for intervention 

a) Monitor at regular frequency: Make close observations to look for early signs of disease and pest presence; use yellow or blue sticky traps to sample for and or control flying insects such as shore flies and fungus gnats

b) Use pest and disease identification tools such as the books and websites listed in the Resources section of this unit (see also Unit 1.8, Managing Arthropod Pests and Unit 1.9, Managing Plant Pathogens). These resources can help with understanding life cycles, seasonal and environmental conditions that favor pests and pathogens, cultural strategies that can prevent or minimize problems, and in some cases, suggest organically approved inputs to use when intervention is necessary.

c) Establish clear tolerance thresholds to initiate control actions, when shifts in cultural practices and environmental management does not provide adequate controls

d) Rogue (cull), or quarantine infected crops to prevent the spread of problems to nearby crops susceptible to the same pests or diseases. Roguing requires sacrificing some for the good of the whole. Quarantining allows treatment strategies to be applied selectively and in isolation from other susceptible crops, thus reducing the likelihood of more widespread outbreaks.

e) As a last resort, use organically acceptable chemical controls, or biological control agents that specifically and selectively target the pest or disease problem you are trying to manage. Following as many as possible of the above strategies and intervening early can greatly reduce losses and increase the efficacy of the inputs organic growers have at their disposal.

f) While most greenhouse pests and pathogens are common across the country because of the similarity of environmental conditions created in greenhouses, speak with local growers, cooperative extension agents, and IPM practitioners to find out what problems to anticipate in your region, which crops may be most vulnerable, the potential severity of particular pests and pathogens, and the times of year to be especially vigilant

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