These days growers have to be much more aware of the pests and diseases associated with the crops being grown. If you can anticipate a problem you are half way to solving it. Fewer chemical controls are available, but the combination of those still listed, together with the way the crop is grown, can be successful. Different plant sectors have specific problems.
Bedding production
Diseases can be a problem, especially early in the spring and in the autumn. Root disease is especially difficult to see before damage is seen on top growth. It is very important to identify the disease as pesticides are very specific and appropriate management techniques can also be different.
Two things are very important to establish once the disease is identified. Firstly the source and secondly how the plant has become infected. For example, if the disease is phytophthora and the source is second-hand trays then it is clearly better to sterilise or replace the trays. Sterilisation of polystyrene trays is almost impossible but plastic can be dipped. Even though the source may be eliminated there is always a chance the floor may be contaminated where previous trays of infected plants have been standing.
As a precaution it is advisable to treat the compost for at least two crops following the problematic one as well as thoroughly cleaning the floor. There are no pesticides that can be incorporated into compost now that Terrazole has been withdrawn. Instead, drenches of products like Filex must be used. This way of treating must be done accurately, ensuring that the compost is evenly wet and the sprayer is calibrated to apply the correct volume of liquid to the volume of compost. Too little pesticide could lead to resistance or no control, and too much may affect the crop.
If the disease is rhizoctonia then a pesticide such as Basilex will need to be used. This disease usually attacks the base of the stem. Thielaviopsis (black root rot) causes black patches on the young roots and Octave is appropriate for control.
A well-drained compost will reduce the risk of all these diseases. The watering technique is also important as most of the above diseases are carried on water droplets. Heavy watering with splashing will rapidly spread the disease spores. Plants are always more susceptible to infection when under stress, as a result of over or under watering.
Aphids are probably the biggest pest of bedding plants. Some biological controls are available. If aphid damage is anticipated then incorporating a pesticide such as Exemptor into the compost can be very effective. Drenching with Intercept 70WG can give good results.
Both these controls are systemic, but there will be a delay between treatment and control as the plant has to be actively drawing water up the stem to the leaves and the strength of the active ingredient will take time to build up to a sufficient concentration to kill the pest. Where attacks are bad it is more effective to give a knock-down spray of pesticide, but the number of products available is very limited and may only be off-label.
pot plant production
Pot plants suffer from most of the problems seen in bedding plants and most of the control procedures will be the same. Poinsettia and cyclamen are very prone to vine weevil attack. Most problems arise through the purchase of untreated plugs where vine weevil eggs have already been laid but go unnoticed. These plugs need drenching immediately with Intercept 70WG before transplanting. If left untreated then the larvae will hatch and frequently either ring the stem or burrow into the stem of poinsettia causing the plants to eventually collapse.
When drenching cyclamen ensure the area underneath the corm is treated. The active ingredient of Intercept 70WG latches on to the peat and will not move unless pulled into the plant via the root. To be fully confident of results, a further drench with the biological nematode, Nemasys, will aid control and will move within the compost.
The pesticide and nematode are compatible. Ensure the compost being used to pot on into is treated with Exemptor as this will control vine weevil eggs and larvae laid at a later stage in growth. Vine weevil adults hide under wood and stones in the daytime. Ensure the glasshouse and area in close proximity to the crop is clear of such debris.
nursery stock production
Nursery stock can have different problems at different stages. Where liners are produced on a floor covered in mypex and phytophthora is a problem, then cleaning the floor and drenching the compost is essential. Just cleaning the floor is not always enough as the disease can get into the sand layer below the mypex. Materials like Jet 5 will not get into this area and when watering the crop the disease will come back up and be spread around.
In these circumstances don’t over water. In persistent cases the sand may have to be replaced. The same diseases affect the roots of nursery stock as those causing problems with bedding roots. The same criteria, controls and management, applies.
The more open and free-draining the compost, the less likely it is that root problems will occur. The down side of this concept is more frequent watering will be required in hot spells. It is essential that adequate supplies are available and irrigation systems are capable of getting that water to the plant.
There is a lot of interest in the use of biologicals for controlling root disease. There are several on the market including bacteria like bacillus and mycorrhiza like trichoderma. This form of control tends to compete for the same food source as the diseases, lowering the potential level and thus the risk of infection. Roots ooze chemicals to deter fungal growth and some of the biologicals do the same thing. The use of biologicals in this way is a form of prevention rather than a cure.
Many of the biologicals have secondary benefits. They work with the plant in a symbiotic way, breaking down organic matter, releasing some nutrient and helping the roots to absorb these nutrients. At this stage it is also worth mentioning compost tea. This product is fermented green waste which, during its maturation, appears to kill any pathogens, but retains some of the mycorrhiza and bacteria and thus acts like the biologicals above. Because its source is plant material, there are also other things mixed in and these include nutrients, particularly potassium, but also many trace elements.
Growth hormones can also be present in compost tea, as well as some of the more complex organic chemicals used to promote growth. Some of these can get into the plant and there are claims that there is a reduced risk of pests and diseases on leaves and shoots. However, the science behind this is vague, but like all new things it may catch up in the future. I say new, but I’m sure organic growers will say they have been using this technique for years and historians will say this was the first form of liquid feed. All things go in cycles.
When potting on nursery stock liners into bigger pots, it is advisable to use compost with controls incorporated such as Vinil, Suscon Green, or Exemptor 10GR. If the liners are more than 12 months old then the compost in the liners should have a drench of Intercept 70WG. If the liners are less than 12 months old and the temperature is above 7C then drench with the eelworm Nemasys before transplanting. Controls do not move within the compost so an untreated liner is open to attack.
Many nurseries are using the biological insects for controlling ariel pests. Care must be taken to ensure controls do not wipe the biologicals out.
There are fewer and fewer contols available these days, so it is essential to use them wisely in combination with biologicals. Management of irrigation, ventilation, and nursery hygiene will also go a long way to reducing the risk of attack from pests and diseases.
There is constant research being undertaken to develop new environmentally friendly controls for pests and diseases. This all costs a great deal of money and entails a very lengthy process. Meanwhile, problems with pests and diseases will continue to be a challenge so every technique available must be used to grow successfully.
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