Category: Knowledge
Date published: February 17, 2023

Top diseases in roses - Powdery Mildew

Just like humans, a rose crop can also be felled by numerous diseases. Which ones strike depends heavily on the circumstances. But every grower must have something in their bag of tricks to defend against fungal diseases such as Powdery mildew

Powdery mildew is a fungal disease that is caused by many different genera of fungi such as Erysiphe, Podosphaera, Oïdium, Leveillula. The main culprit in roses is Sphaerotheca pannosa. This fungus penetrates the plant cells and absorbs nutrients, leading to severe consequences: the plant weakens and seriously damaged leaves turn yellow and fall off. A serious mildew infestation can have dire consequences for production.

It's essential to take action as soon as the first symptoms appear, otherwise it will be difficult to get the situation under control.

Life cycle and appearance of Powdery mildew

The fungi that cause powdery mildew (Erysiphe, Podosphaera, Oïdium, Leveillula) are all biotrophic fungi, meaning they feed on living plant cells and barely survive in the absence of a living crop. They survive as ascospores or perithecia, structures containing ascospores.

Fungal spores germinate on the leaf surface and the germ tubes grow and branch out on the leaf surface. Small structures called haustoria are produced, from which the fungus penetrates the plant cell and takes up nutrients from the epidermal layer of plant cells. Most of the fungus remains on the outside of the plant surface.

On the mycelium on the plant surface, new conidiophores are formed, structures which contain new spores, the conidia. These conidiophores are the fluffy fungal growth so typical of powdery mildew.

Conidia are the main means of dispersal. They are dispersed by wind, as are the ascospores. In general, powdery mildew fungi are not very resistant to water, so free moisture often limits epidemics. Spore dispersal by water is negligible since spores explode when in contact with water for no more than a few hours.

Powdery mildew spores, either ascospores or conidia, don’t need nutrients or water for germination. In general, the development rate of the epidemic is fastest at 18-25 °C. High relative humidity (RH) promotes the germination of spores but inhibits spore production, so the net effect of the RH is very small. Wind and draught in greenhouses promote disease. In greenhouses, the dispersal of spores over small distances (limited time) mostly happens via workers' activity while working in the crop

Damage symptoms

Powdery mildew symptoms are fairly similar in all plant-pathogen combinations. The symptoms are white fluffy colonies, first and foremost on the upper side of the leaf. In severe epidemics in cereals, the ears may also be infected.

In most crops, the white colonies later turn brown or grey. Severely affected leaves may turn yellow, curl up or drop off.

How to manage Powdery mildew

  • Use resistant cultivars whenever available
  • Extra Silicon and/or Calcium nutrition hardens the cell walls and makes it harder for the fungi to enter the leaves
  • To reduce the chances of powdery mildew in greenhouses, it is advised to first ventilate by only opening the screen curtains on the lee side.
  • In fruit trees, pruning should be done early in the season
  • Do not provide excess nitrogen since this promotes the disease
  • Prevent plant diseases by optimizing plant potential and crop resilience.
  • Use alternative biorational solutions for preventive and curative control of powdery mildew

Solutions we offer

Koppert offers Veni Biosulfur, a novel biorational product that contributes to building the resistance and immune system of the plant. It provides an essential building block for the formation of amino acids, proteins, enzymes, antioxidants, and vitamins, which gives rise to better biological activity. The resistance and the plant's immune system are strengthened and the crop becomes healthier and more resilient. Overall, the plants’ tolerance, and resistance to powdery mildew are enhanced.

Talk to a Koppert consultant in your area to guide you on the integration of biorational products in your management strategy for powdery mildew.