Taking Control

The way in which pests and diseases in horticulture are controlled is changing. This is not necessarily because growers want to change, but rather new regulations dictate that pesticides are regulated more closely than in the past. Due partly to cost and environmental constraints, many of the chemicals that were available are now no longer supported. As a result growers now have to make the most of the few pesticides available and integrate them with other means of control including environment management and biological methods.

Pests and diseases have to be split for control purposes and these can be further split into problems that can be seen above ground and those hidden below ground.

Pests Below Ground

Probably the most important pest that causes damage to roots is the Vine weevil. The adults are all female and lay eggs from about June until late September outdoors in a normal year. Under warmer protected conditions the egg-laying season may be longer. The adults are extremely difficult to find and control as they are nocturnal and hide in hedge bottoms, under pots, stones and similar areas.

Deterring egg laying is possible using pot toppers or very coarse material on the pot surface. This will also deter weeds so it has a double benefit. There are pesticides like Intercept, Fipronil, and Suscon Green which all control the eggs and young larval stages very well. Pesticides even work in cool temperatures when other methods fail. Nematodes work well in warmer temperatures, but the two are compatible so it is worthwhile using them as a combination. Newer strains of nematodes are being developed for colder conditions.

Sciarid fly can also be troublesome especially in slow growing plants that do not cover the compost surface. The same controls described for Vine weevils can be used successfully, but management of the crop is also important. Sciarid flies are attracted to dark, wet compost that is high in organic matter. Having well-drained compost and dry pot surfaces is an enormous benefit. Don’t use bigger pots than necessary as this leaves a lot of uncovered compost.

Pests Above Ground
These pests can generally be seen or, if not, the damage they cause can identify them. There is a vast array of biological insects that can be used to keep most of these pests under control in the warmer months. However there will always be pockets that get away and a lot of damage can occur before biologicals can catch up. In these circumstances pesticides are essential.

Where possible spot treatment should be used, as many pesticides also kill biological predators, so care must be taken. Use a systemic insecticide applied as a drench as this has less effect on predators. There is no point in building up a costly biological control and then killing them off and having to start again. Management-wise, a strong vigorous growing plant will be more able to tolerate a low level of pests. Keep weed species around the nursery to a minimum, as many can be hosts to the pests.

Diseases Below Ground
There are a large number of diseases that can attack roots and most are naturally in compost or in the nursery all the time just waiting for the right conditions to enable them to get into the plant. These diseases include pythium, phytophthora, rhizoctonia, fusarium, and thielaviopsis.

It is very important to identify which disease is causing the problem if pesticides are to be used, because different disease groups are controlled by different pesticides. For example Filex and Terrazole will control pythium and phytophthora but not the others. Basilex will control only rhizoctonia and Octave will only control thielaviopsis and fusarium. Putting the wrong one on will have no effect and is a total waste of money.

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 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 pest and diseases on leaves and shoots. However, the science behind 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, or so the saying goes.

In terms of management most root diseases can be avoided by having very open compost and keeping watering to a minimum. Water is the most common means of transporting disease spores and puddles are a huge source of contamination. A vigorous plant is less likely to succumb to diseases, whereas a stressed plant will almost certainly become infected.

Diseases Above Ground
Many diseases infecting leaves and stems are a result of environmental conditions, such as botrytis, which can be avoided by keeping the air dry and preventing static air. Air movement is very important. In the summer this can be achieved through ventilation, but in the winter fans can do the same job. The aim is to prevent a film of water forming on the leaf surface, as this is where spores germinate. If they can’t germinate then no infection can occur.

Identification is again essential where pesticides are to be used, as many are species specific. There are many all-round pesticides that will control fungal leaf spotting, but there is no control for bacterial leaf spotting. Systemic pesticides are best, as they tend to prevent and cure. Pesticides that act on the leaf surface as a preventative are less environmentally friendly, as they have to be applied regularly in anticipation of problems that may never occur.

I know of no biological methods of control for these above ground diseases.

Conclusion
In any risk assessment the first operation is to identify the possible pests and diseases that might affect the crop being grown and assess where the source might come from. The second operation is to grow the crop in such a way as to minimise the risks. This might mean more open compost, reduced watering, better ventilation and air movement, and prevent crop stress. If this fails and pests and diseases are seen, then the third operation of control comes into play.

Biological control is always the most environmentally friendly method, but in practice a combination of biological and pesticides is usually best. Biologicals can be slow, so initial pesticide use to get everything under control followed by biological methods quite often gives the best results.

Pesticide manufacturers are striving to come up with new environmentally friendly products and in the next 5-10 years more will be on the market. Biological forms of control are in vogue and the fastest moving change will be seen in this area. New species and new strains of existing biological agents will be seen, including ones that will work in cooler temperatures. The research being undertaken is very exciting and there will constantly be new products being released to help growers produce the highest quality plants with reduced pest and disease problems.