Practical steps to disease-free crops

A look at cultural controls to counter pathogen spread.

With a restricted range of fungicides, and the need to use them sparingly to reduce the risk of resistance occurring, it makes sense if your primary means of disease control is to manage the nursery environment.

The key is to know the conditions that favour the spread of the pathogens most likely to attack your crop, and then manage the growing environment to avoid them. But you also need to adapt your strategy to the conditions — last year’s hot, dry summer favoured a different combination of pests and diseases to this year’s long periods of rain. So far this year, powdery mildews have been the biggest disease problem. Awareness of the need to ventilate tunnels and space crops to aid air movement means growers have generally seen less Botrytis than might have been expected.

Crop health

An integrated disease-management strategy starts with ensuring the crop is physically healthy with adequate nutrition for strong growth without over-feeding; and adequate irrigation without over-watering. Avoid accidental damage to roots and shoots that could be entry points for pathogens such as cankers and grey mould. When potting, handle the plants carefully and avoid potting too deeply. Take care when tying climbers or trees to canes or stakes and consider chemical defoliants to ensure healthy abscission wounds at autumn leaf fall.

When propagating, ensure material is only taken from pest- and disease-free mother stock — ideally these are dedicated stock plants growing in an area where you can manage them separately and limit the number of people handling them.

Consider a preventative fungicide programme on stock plants, both to prevent infection and to avoid cuttings carrying contamination through into the propagation area. Try to protect the shoots used for cuttings production against contamination through rain splash from the soil or other plants and clean and disinfect propagation tools between stock plants or batches.

Although compost teas and biostimulant products tend not to perform well in direct comparison trials with chemical fungicides, there is at least anecdotal evidence that they can help strengthen a plant’s natural defences against pathogens and pests. While there is still much to learn about how they work, compost teas or biostimulants are being used by growers, who report fewer disease problems and less fungicide use.

Both stock plants and crops should be monitored for pests and disease regularly through the year and more often in the growing season. Ensure staff are trained to recognise early signs of plant stress and the key pest and disease symptoms, and about any pests and diseases to which each crop is susceptible. Ensure monitoring includes the root-zone as well as shoots and leaves.

It can be useful to keep a pest and disease diary based on your regular crop walks and recording of outbreaks. This can help you identify trends and, for example, to get to know vulnerable locations on the nursery. All of this can sound like unnecessary work but it helps you anticipate problems so they can be easily nipped in the bud — waiting until an outbreak takes hold means it is more difficult and time consuming to deal with and the risk of crop loss is higher.

Nursery hygiene

Fungal pathogens can build up on dead and dying plant material, creating a source of inoculum that puts the crop at risk, so ensure such material is regularly cleared from the nursery. An HDC project (PC/HNS 121) showed that picking over a crop such as Cyclamen to remove the dead leaves at the plant base was as effective as a full fungicide programme in preventing Botrytis. While that may not be cost-effective as an individual treatment it illustrates well the importance of hygiene.

Weeds can act as a significant reservoir of pests and pathogens so good weed control is essential. Some of the large plant propagation nurseries in the US have a standing instruction to all staff to pull any weed on sight. However, a balance needs to be struck, as weeds such as nettles can provide food and shelter to beneficial insects that can help keep down pest populations.

Thoroughly cleaning beds or benches between crops helps suppress the gradual build-up of a population of disease spores to the point where they can establish on the crop. It also reduces any risk of carry-over of disease from one crop to the next.

Regular cleaning and disinfection of tools and kit reduces the risk of spreading both diseases and pests such as nematodes. Don’t mix compost on the ground where it can pick up fungal spores. Store it under cover and where it can’t become contaminated from any run-off water — the ground should slope away from the store. Storage for pots, trays and anything else likely to come into contact with the crop should also be protected from contamination.

Irrigation water from rivers, open reservoirs or recycled from nursery beds should be cleaned before being applied to the crop, and any clean water should be stored in covered tanks. Slow sand filters have been shown to remove or kill spores of even the most persistent diseases such as phytophthoras (eg HDC project HNS 134).

Biosecurity

This is about avoiding bringing pathogens onto the nursery. Ensure you only buy in plants, cuttings and seeds that you can be confident are disease free. If the material doesn’t have a plant passport, you should obtain assurances from your supplier about their own plant health measures.

However confident you are about the source of bought-in plants or cuttings, always inspect consignments carefully. It is a good idea to establish an area where these can be kept so that if symptoms do appear shortly after arrival the problem is less likely to spread to the rest of the crop — the consignment can then be treated or destroyed as necessary. Botrytis, in particular, can remain as a symptomless (latent) infection for many weeks. An established quarantine procedure may also help you if you need to prove the source of an infection.

The advent of diseases such as Phytophthora ramorum in the nursery stock sector and impatiens downy mildew in glasshouse ornamentals has made nurseries more aware of biosecurity. But many specialist propagation nurseries, particularly those involved with crops such as vegetatively raised bedding or pot plants, operate very strict hygiene practices and nursery stock growers in the US and Australia have begun to pick up elements of these. For example, disinfectant footbaths can be found on nurseries in California, either to protect areas of a nursery where particularly vulnerable crops are grown or at main entrances for all employees and visitors.

On nurseries propagating vegetatively raised bedding plugs, workers wear aprons and gloves, changed at least after every break and more often if they are working in particularly vulnerable crops. Many nurseries have also installed handwash stations using water-less cleansers — workers and visitors have to clean their hands before handling plants. Insect mesh is used to keep pests out of glasshouses. Many nurseries also operate strict no-smoking policies, as tobacco can carry plant viruses which can be transferred to a crop on workers’ hands.

Environmental management

Research funded by HDC a few years ago (PC/HNS 121) showed just how important environmental management is in reducing disease risks. That particular project focused on Botrytis, which appears to have been far less of a problem this year than it might have been, given the recent run of wet or humid weather, because so many growers have taken on its recommendations.

The project proved the importance of air movement around the crop to stop humidity build-up. The critical threshold for Botrytis was 95 per cent for three hours. If you can avoid that, there should be little risk of the disease developing even if humidity is high for shorter periods. However, it is the humidity within the crop canopy that counts.

In unheated tunnels, opening doors and sides and using fans helps to keep the air circulating. Researchers Mark McQuilken of SAC and ADAS principal plant pathologist Tim O’Neill recommended spacing plants to avoid foliage contact.

Greenhouses should be ventilated early in the morning, recommends an HDC factsheet (23/02) based on the project.

Covered propagation beds or benches should also be ventilated regularly and misting reduced as soon as rooting begins.

Most growers are now aware that badly managed overhead irrigation can create ideal conditions for disease spread.Sub-irrigation eliminates these problems and should be considered when installing new beds — not just for disease management but for their water efficiency. But if you need to use an overhead system, time applications so that foliage has time to dry at the end of the day. Don’t forget that it is possible to over-water using sub-irrigation, which can lead to root diseases or enable Botrytis to infect around the lower stem.

Role of fungicides

It is important to integrate cultural controls and preventative measures with fungicides. The development of fungicide resistance in plant pathogens is an ever-present risk, which the use of good cultural controls helps to reduce because you are relying less on the fungicides.

But resistance management also includes using fungicides with different modes of action in a planned programme; and timing and targeting applications to specific risks. Reducing the dose rate or failing to apply thoroughly encourages the evolution of resistant strains. Remember, some fungicides may affect the performance of compost teas.

Cultural control for root diseases


• Inspect all incoming consignments of young plants and monitor crops regularly.
• Avoid spreading spores when disposing of infected plants.
• Control sciarids and other insects, which may spread spores, and control root-zone pests, which may spread spores or wound roots to create infection points. Control weeds, which may harbour wilt pathogens.
• Use disinfectants between crops or at your end-of-season clean-up to control spores surviving on surfaces or in growing media dust on tray filling and potting machines.
• Sterilise re-used pots or trays or use new ones for each crop.
• Prevent build-up of plant debris or growing media dust.
• Use well-aerated growing media and avoid high moisture.
• Spores can be dispersed by water splash so consider sub-irrigation.
• Clean all recycled water.
• Cover stored growing media and treated irrigation water to reduce entry of wind-blown or insect-carried spores.
• Maintaining root-zone pH below 5.5, if possible, can reduce severity of black root rot.
• In pot plants, fusarium and verticillium wilts may spread faster on ebb-and-flood benches than on capillary matting.
• Growing media containing bark or woodfibre or crushed shells may help to suppress wilt.
• Do not pot too deeply and avoid root damage during potting or transplanting, and when handling and transplanting.

For shoot and leaf diseases

• Quarantine and inspect plant deliveries and monitor for outbreaks.
• Consider spectral filter claddings (eg UV absorbing or reflecting) to minimise sporulation and slow spread of outbreaks.
• Avoid damaging plants, potting too deeply, or conditions that may lead to physiological or mechanical damage such as scorching.
• Keep plants well spaced and ensure good air circulation and ventilation.
• Avoid over-watering and, if using overhead irrigation, time applications so plant surfaces will dry quickly.
• Consider using sub-irrigation to avoid wet leaves and reduce humidity.
• Pick over plants for spent flowers, dropped petals and senescing foliage, and remove plant debris from benches and beds.
• Control weeds, which can be a source of overwintering infection.
• Wash down and disinfect between crops.


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