| ||Crop monitoring | Cultural control | Resistant cultivars | Biological control | Chemical control
Successful crop production requires that crop pests and diseases be managed so that the effects of diseases and pests on the plants are minimized. The management of crop diseases is directed at preventing the establishment of diseases and minimizing the development and spread of any diseases that become established in the crop. Managing pest problems is directed at preventing the pest populations from becoming too large and uncontrollable (Portree 1996). The presence of pests and diseases are a fact of crop production and growers must use all available options and strategies to avoid serious pest and disease problems.
Integrated pest management (IPM) is a term used to describe an evolving process where cultural, biological, and chemical controls are included in a holistic approach of pest and disease control (Howard et al 1994). Key components of effective pest and disease control programs include: crop monitoring, cultural control, resistant cultivars, biological control and chemical control (Howard et al 1994, Portree 1996).
Crop monitoring is the continually on-going surveillance to detect the presence of a pest or disease at the very early stages of development of the disease or pest population, before economic damage has occurred (Howard et al 1994). Everyone involved in working the crop should be made aware of the common pest and disease problems and what to look for to detect the presence of problems in the crop. In addition this general surveillance of the crop, dedicated monitoring of the crop should be included in the weekly work schedule (Howard et al 1994, Portree 1996). Blue sticky cards, placed throughout the crop, are a useful monitoring tool to help trap and detect pest problems before they become a problem (Howard et al 1994). Yellow sticky cards are known to attract and catch some biological control agents e.g. Aphidius sp. (Don Elliott, pers comm) Biological control agents can be released well in advance of any pest population explosion thus allowing for the establishment of the control agents and prevention of a serious pest problem.
Crop monitoring should begin when the crop is still on the seedling table or at the transplant stage (especially when transplants are obtained by another greenhouse ie. purchased from a propagator) (Portree 1996). If transplants are being purchased from a propagator it is important to be in contact with the propagator regarding any pest problems encountered during the production of the transplants. It is also important to know what pest control measures were used, if any, to control the problems. It is advisable to establish in advance, what control measures you are willing to have applied to the transplants at the propagator's prior to receiving them into the greenhouse. The concern is that any pesticides that are applied are compatible with the pest control programs i.e. biological control programs that will be used for the duration of the crop. Some growers may insist that only biological controls be used during the production of transplants, and/or that biological control agents be introduced preventively to the transplants before they are received.
Cultural control involves providing the conditions that favour the growth, development and health of the crop, and where ever possible, providing conditions that work against pest and disease (Howard 1994, Portree 1996). Many disease causing fungi and bacteria require the presence of free water or condensation on the plants in order to cause disease (Jarvis 1992, Howard 1994, Portree 1996). High relative humidity promotes the development of disease, and maintaining the environment below 85% relative humidity will help to escape or avoid disease problems (Jarvis 1992, Howard 1994, Portree 1996). Ensuring proper ventilation and air movement within the crop canopy, as well as maintaining optimum plant spacing and a relatively open canopy, will ensure good air circulation and minimize the establishment of micro-climates that favour disease development. Proper contouring of the greenhouse floor will avoid the pooling of water which contributes to localized high relative humidity. Optimizing the greenhouse environment to favour the development of the plant will ensure a strong, healthy plant which is not only a prerequisite for high yields but also results in plants that are better able to resist diseases and insect pests (Jarvis 1992, Howard 1994, Portree 1996).
Good crop sanitation is another important component of successful cultural control. The plants must be pruned and maintained on schedule, all crop debris should be promptly removed from the vicinity of the greenhouse. Any weeds that happen to gain a foot hold through gaps in the floor plastic should be removed immediately upon discovery and the floor repaired. Personal plants "pet" plants should not be grown in the greenhouse. Both weeds and "pet" plants can be as source and "haven" for pest and disease problems.
Pruning tools and other equipment should be cleaned and disinfected on a regular basis. Aprons or other clothing worn by the workers should be washed frequently. When a disease or pest problem area exists in the greenhouse, that area of the greenhouse should be worked last, to avoid the spread of the disease or pests by the workers. In this situation, special care must be taken to disinfect tools and to clean clothing.
Maintain a 6 to 10 meter wide buffer zone (Howard et al 1994, Portree 1996) around the outside of the greenhouse by regularly mowing any weeds that try to grow in this zone. The presence of plants in close proximity to the greenhouse can serve as a reservoirs for continual introduction of pests and diseases into the greenhouse. Screening of the intake vents can also play an important role in excluding pests from the greenhouse. It is not enough just to screen-off the intake vents as the screening restricts the air flow into the greenhouse, it is important to ensure that the surface area of the screening used is large enough so that it does not restrict the flow of air into the intake vents (Chang 1996). This may require that a screen 'chamber' be constructed over the vent.
Plant breeders have had considerable success in developing cultivars that contain genetic resistance or tolerance to diseases. When selecting the cultivars to be grown, it is important to consider the genetic resistance of the cultivars to the prevalent disease problems in the region (Howard et al 1994). The development of cultivars possessing genetic resistance to pests has been relatively unsuccessful (Howard et al 1994), however, the techniques of genetic engineering have made inroads in conferring pest resistance in plants. Genetically modified, pest resistant plants may become available to greenhouse growers in future. The development use of genetically modified plants or genetically modified organisms (GMOs) is currently a contentious issue, and may not be accepted by growers or consumers.
Biological control uses beneficial organisms, primarily predators and parasites, to control pest populations below economically important levels. The goal is to establish a balance between the pest population and its parasites and predators to keep the pest population under control. The complete eradication of the pest population is not the goal of biological control programs, as some pest organisms are required so the parasites and predators can reproduce.
The greenhouse industry has a well established reputation for using biological pest control agents more than any other crop production industry. The reason for this is, in part, due to the ability of greenhouse growers to manage the environment to favour the biological control agents. Another factor is the relatively limited number of pest species in greenhouses, as well as a general tolerance of greenhouse crops to leaf damage caused by these pests. The high value of greenhouse produce is another reason why the use of biological controls is economical in greenhouse crops. The increased use of biological controls has led to a reduction in pesticide applications as the industry leads in environmentally responsible, intensive crop production.
Effective biological control of diseases is a more difficult goal and to-date, has rarely been achieved (Howard et al 1994). However, research in developing biological controls for greenhouse crop diseases is ongoing and it is likely that biological control products for greenhouse diseases will be available in Canada in the near future. The primary strategy of biological control for greenhouse plant diseases is to introduce fungal parasites to control populations of disease causing fungi in the greenhouse environment so that they are unable, or have a reduced ability to infect the plants. Some of the promising biological control agents, for example, fungi in the Genus Trichoderma are also strong competitors of the disease causing fungi such as Botrytis cinerea, and can be used to protect wound sites to prevent Botrytis from colonizing the wound site.
Pesticides are valuable tools when used as a component of an integrated pest management program (Howard et al 1994). Insecticides should be applied only in support of biological control programs, dealing with localized pest outbreaks in the crop that have escaped the biological control agents. When insecticides are used, care must be taken to ensure that they are compatible with biological control agents, that there will be minimal long term adverse residual effects on biological control programs. Fungicides are used only when a disease problem is detected.
Pesticides are regarded as the controls of last resort because their misuses creates high-profile environmental and food safety problems (Howard et al 1994). Also, the application of some pesticides to a crop can cause stresses that reduce the productive life of the crop and can make the plants susceptible to other pests and diseases (Howard et al 1994). If the use of biological control agents is to obtain a balance between pests and predators that does not threaten the productive yield of the crop, the indiscriminate use of pesticides creates imbalance and uncertainty in the crop.