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Integrated disease management in greenhouses

Integrated disease management is the practice of using a range of measures to prevent and manage diseases in crops. Hazard analysis is used to identify the potential for infection so that preventative or curative measures can be put in place to minimise the risk of disease infection and spread. During the cropping cycle, regular crop monitoring is used to decide if and what action is needed.

Today, the term integrated pest management (IPM) is used to describe the use of integrated practices to manage any kind of pest including diseases. IPM is fundamentally a subset of the good agricultural practices needed to produce profitable and productive crops in a sustainable way.

A disease is a condition in a plant that affects the plant’s normal functioning or development. For a disease to occur, three conditions must be met. Firstly, a pathogen has to be present on or in the plant. Secondly, there needs to be suitable environmental conditions for the pathogen. Thirdly, the plant must be susceptible to the disease.

Disease management

We know that in geometry, three points form a triangle. In disease (and pest) management, if we imagine that there are three key points that determine control, then we in effect have a disease (and pest) control triangle. If we can control one or more of the three points, then a pathogen (or pest) can be controlled. For example, a disease can be controlled by removing the pathogen with hygiene or a pesticide, changing the environmental conditions to one that does not favour the pathogen or using of resistant/tolerant variety so that there is not a suitable host.

Hygiene

The most effective way to manage diseases is to prevent the pathogens from getting to the crop. Make sure that any materials, containers, or equipment that you bring into the greenhouse is clean. For more information refer to Preventing pests and diseases in the greenhouse.

Install and maintain a foot bath at every entrance to the greenhouse. Although commercial foot bath pads are available, a container with a piece of foam and disinfectant solution is effective. Make sure every person entering the greenhouse uses the footbath each and every time that they enter and that the disinfectant solution is changed at least every fortnight. It will be necessary to change the solution more frequently if it looks muddy.

Note that it may not be possible to keep all pathogens out of a greenhouse – some diseases of greenhouse crops first appear on plants under the vents because airborne spores have been blown into the greenhouse. Despite this risk, good hygiene will significantly reduce losses caused by disease.

Another important strategy is to require all workers who smoke to wash their hands thoroughly with medicated soap before entering the production area. This is because some viruses and bacteria can be passed to plants just by touch.

If it is feasible, empty the greenhouse completely between crops and clean the structure. Do not use formalin in the greenhouse because the fumes can damage plants and they are also hazardous to workers. Growers with clean greenhouses have significantly fewer problems with diseases.

Control entry

Control access to the greenhouse. It is important to understand that pathogens (and pests) are easily carried on clothing and shoes. Many diseases in greenhouse crops first appear near doorways. The fewer people entering the greenhouse, the smaller the chance that pathogens (and pests) will be carried into the crop.

When people are visiting, have them wear disposable overalls. Avoid having visitors who have come directly from another greenhouse. If visitors and workers are moving between different crops, always move from the youngest and healthiest crop plantings through to the older crops (that may be infected) to reduce the risk of spreading pathogens.

Start with disease-free plants

If buying seedlings, inspect them upon delivery. If any plants have disease symptoms, tell the delivery person immediately. Remove the diseased looking plants and put them in sealed plastic bags and submit them for diagnostic testing. Have a special designated clean area or greenhouse to store seedlings in prior to transplanting out. Before moving new plants into the greenhouse, check them for any sign of pests or diseases. Do not plant out any plants that are, or appear to be, diseased or infested with pests.

Choose crops and varieties that are resistant to pests and diseases if feasible. Many cultivars are resistant or tolerant to several diseases including Powdery Mildew, Downy Mildew and viruses. However, no cultivar is resistant or tolerant to all diseases.

Control the growing environment

Controlling the greenhouse environment to make conditions less favourable for disease organisms is a very effective method of disease control. Good temperature and humidity management are essential to minimising disease in greenhouse crops, particularly for Downy and Powdery mildews and Botrytis. Guttation is an important way in which pathogens can infect greenhouse crops. When guttation occurs, pathogens are picked up off the leaf surface by the exudate from the leaf and then can be sucked into the plant during the day.

Condensation on leaves, tissue damage such as pruning and picking wounds, chilling injury or heat damage predisposes plants to infection.  Condensation can also affect fungicide activity by diluting fungicide applications. Over time, this may actually contribute to the development of resistance in pathogens to particular pesticides.

Inspect plants regularly

Monitoring the crop regularly will enable early detection of diseases. This greatly improves the efficacy of control strategies. Walk up and down every row and inspect at least 5% of all plants in the crop. Some diseases will occur in certain locations in a greenhouse because of localised “microclimates”. For example, Powdery Mildew usually starts in the shadier areas.  Botrytis occurs where moisture sits on plant surfaces. These specific areas can be targeted when monitoring the crop to find diseases at an early stage of development. Tie coloured plastic tape or ribbon in these areas to signify “hot spots” for diseases. If required, these areas can be "spot sprayed" with a small compression or backpack sprayer rather than needlessly treating the whole crop.

Waste management

Remove and destroy crop residues as soon as possible after pruning and harvest. Do not pile plant material near the greenhouse. Put pruned material directly into bags or a rubbish skip bin for disposal – not on the ground. Make sure the skip bin is removed regularly to avoid a breeding place for pathogens. Crop debris can be buried if done immediately. Do not stockpile it. Burning crop debris may contravene Local Environmental Plans. Check with your local council.

Control insects and weeds

Control insects and weeds inside and outside the greenhouse. Weeds can harbour diseases and pests. Insects can carry diseases. If feasible, place insect screens over all openings in your greenhouse. Be aware that screens reduce the flow of air and will impact on the venting capacity of the structure.  Poor air circulation can result in diseases such as Botrytis (grey mould), Alternaria (leaf spot) and Pseudoperonospora (Downy Mildew). The use of a double-door entry to the greenhouse, with a footbath, greatly reduces both pests and diseases getting into the crop.

Fungicides

There are two different types of fungicides used to manage diseases – protectants and eradicants.

Protectants sit on the surface of plants and their mode of action relies on contact with pathogens for control. As new growth needs to be protected, growing plants need on-going protectant spray applications. These chemicals generally control a wide range of fungal pathogens. When using protectants, make sure plant coverage is thorough and even.

Eradicants or curatives are systemic pesticides and are absorbed by plants. Their mode of action controls pathogens at sites some distance away from where the chemical droplets land on the plant. These chemicals may move into new growth and therefore do not need to be applied as often as protectants. Unfortunately, because systemic pesticides are specific to the type of fungi they control, their continued use can lead to resistance developing in the fungal population. This has occurred with Downy mildew, Powdery mildew and Grey mould for example.

Both bacteria and fungi spores of pathogens can be spread inside the greenhouse with a moving spray mist created when applying pesticides.  If the pathogen is resistant to the fungicide, the act of spraying can make the problem worse. Copper fungicides have a reputation of failing because of this situation.