In order to test the influence of existing interventions on the natural defenses of a plant, various climate chamber tests have been carried out by researchers within the Mildew Programming Study with variation in daylight sum, CO2 and RH. After three weeks exposure time, the influence on the production of defense proteins, which are a measure of induced resistance, is very small. However, it is confirmed that the RH and light sum do influence the mildew development more strongly than the CO2. In addition, the researchers believe that the following of changes in the nutritional composition of plants may be important in order to get, in an early stage, a picture of a change in the sensitivity of the crop for a fungal disease. Sodium appears to be an important element in this.
Several pillars have been distinguished within the system approach of mildew. In the first part of the research, a start was made with generating new knowledge about the effects of existing interventions on the natural defense system of the crop which play a role in mildew, and these have been tested in controlled climate chambers.
Natural immune system
In the second part, new knowledge is generated about the effects of new interventions on the natural defense system of the crop. To test the influence of existing interventions, and in consultation with the sounding board group, various tests were carried out with climatic factors such as light, CO2 and relative humidity. After three weeks of exposure time, no significant influence can be seen on the production of defense proteins that are naturally produced by a plant itself. Only in potted gerbera the production increases slightly at 90% relative humidity and at a CO2 content of 900 ppm a slight trend appears to be visible that production is reducing slightly. The influence of existing climate factors on the flexible, induced resistance therefore appears to be very limited.
Control factors
Controlling on an increase in defense proteins can be achieved more effectively with other control factors, such as the use of synthetic elicitors or through specific control of light quality with different red: far-red ratios. Although the influence via direct control on plant resistance is small, the climate can still be used to manage the reduction of the sensitivity for mildew. The impact of relative air humidity and light sum are thereby of greater influence than the amount of CO2.
The tests show that following changes in the nutritional composition of plants may be one of the first indicators that the sensitivity of the crop for a fungal disease is changing. In any case, sodium seems to be an important element to keep an eye on.
A promising new intervention to reduce the sensitivity for mildew is to control a nutrient operation with more MgCl2. Additional research is needed to further develop these applications for the different crops in greenhouse horticulture.
Recommendations
The contribution to mildew control from a resilient plant should primarily be sought in the optimization of climate conditions that have a direct effect on the traces and this can then be supplemented with resilience products that specifically enhance the induced resistance of a plant.
- More research is needed into the preconditions under which the different mildew species that occur on gerberas can germinate well. The traces of the Podosphaera variety seem much more sensitive to exposure to free water than the Oidium varieties. For effective control it is important to know which varieties are dominant at the nurseries. Podosphaera varieties seem, by maintaining a relatively high air humidity, to be getting weaker. However, a humid cultivation climate seems to strongly promote the growth of Oidium varieties.
- Further development of molecular detection of mildew in greenhouse horticulture crops, can contribute to a better understanding of sensitive development phases in the interaction with the plant.
- Attention to the relationship between plant sap analyses and disease sensitivity. For example higher sodium levels appear to correlate with lower sensitivity for mildew. Possibly plant sap analyses may be used as an early indicator for disease sensitivity.