Powdery mildew (PM) is a widespread fungal disease that affects a range of plants including grapes and other fruits, cucumbers and a variety of vegetables, flowers like rose and chrysanthemum, and medicinal plants like cannabis. Here are eight scientifically proven conditions that intensify powdery mildew infections and cause the disease to spread from plant-to-plant.

1. Humidity

Humidity is strongly correlated with PM disease severity. Areas that are humid, have low air circulation, or have stagnant water are most likely to have PM infections [1, 2]. Humidity between 80 – 90 % is most favourable for spore germination. However, levels between 50–70% can still increase risk. Humidity of 50 – 70 % can increase spore germination and disease development [2]. In contrast, low humidity (20 – 40 %) has been shown to prevent spore germination and PM growth [2].

2. Water Splashes from Irrigation

Leaf wetness and water movement can contribute to the spread of PM. Watering plants from above can result in splash droplets moving from leaf to leaf. As a result, these water droplets disperse spores. Just three raindrops are needed to release 53% of spores from one plant to another [3]. Ensure that leaves have a chance to thoroughly dry between waterings.

3. Air Flow and Leaf Movement

Wind speeds as low as 2 – 3 m/s instantly disperse PM spores [3]. To give context, this wind speed is only strong enough to rustle leaves, but not enough to cause constant movement. For this reason, if a facility has PM in one area, growers may wish to slow or completely stop fans to prevent spores from moving around.

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4. Temperature

While humidity and moisture are the key contributing factors, temperature should not be ignored. Temperatures between 20 – 21 °C increase disease progression [2]. In contrast, temperatures above 28°C will slow PM spore germination, germ tube elongation, and disease development [2]. Short periods (2 to 3 daily exposures of at least two hours) of high temperatures are sometimes enough to suppress disease development [2]. To completely kill PM, longer and more intense heat treatment is needed (2 – 4 days at 28 – 35 °C). Unfortunately, these temperatures will have a negative effect on plant health and flowering [4, 5].

5. Irrigation

Studies show that doubling the amount of water plants get makes PM worse by 2 to 7 times [6, 7]. Increased irrigation levels are thought to increase PM severity by maintaining leaf temperatures in a range that is conducive to pathogen growth [6, 7]. The increased irrigation allows for increased evapotranspirative cooling. Irrigation may also increase the microclimate humidity.

6. Light Quality, Quantity, and Duration

The type of light used in cultivation impacts PM disease severity. Compared to natural light (i.e. the sun), plants grown under HPS lamps tend to have more fungal spores [8]. This difference is likely because sunlight has UV-B rays, whereas artificial lights do not. Light quantity (PPFD) also has an effect on PM levels. Lower light intensities (like shading in a dense canopy) can increase PM disease severity by 70% to 275% [9]. This was tested in pepper plants using shade nets to block light. The duration of light can also make a difference: one experiment with rose showed that 20 – 24 hours of lighting strongly reduced PM severity compared to 18 hours or less [10].

 

7. Plant Density and Leaf Removal

Canopy management practices that allow more light into the plants can help prevent PM [7, 11]. Very shaded canopies (with < 10 % of light) have 2 – 4 times more PM compared to lightly shaded canopies (50 % of light; [6, 7]. Two ways to let in more light is by Scrogging or trimming leaves. Research shows that removing basal leaves 2 weeks after flowering may help with reducing PM [6, 7]. In contrast, removing leaves 5 weeks after flowering has no effect on PM levels [6, 7].

8. Colony Age

Older PM colonies disperse more easily than younger colonies. In other words, the longer you have a PM infection, the higher the risk of spread. 24-day old colonies disperse more spores than 12-day old colonies.

Learn More

  1. Carroll & Wilcox (2003). Effects of humidity on the development of grapevine powdery mildew. Phyto., 93 (9), 1137–1144.
  2. Guzman-Plazola, R. A., Davis, R. M., & Marois, J. J. (2003). Effects of relative humidity and high temperature on spore germination and development of tomato powdery mildew (Leveillula taurica). Crop Protection, 22 (10), 1157–1168.
  3. Willocquet, L., Berud, F., Raoux, L., & Clerjeau, M. (1998). Effects of wind, relative humidity, leaf movement and colony age on dispersal of conidia of Uncinula necator, causal agent of grape powdery mildew. Plant Pathology, 47(3), 234–242.
  4. Quinti, J., & Jr, С. P. (1982). Effects of temperature, light, and relative humidity on powdery mildew of begonia. Phtypath.
  5. Sage, T. L., Bagha, S., Lundsgaard-Nielsen, V., Branch, H. A., Sultmanis, S., & Sage, R. F. (2015). The effect of high temperature stress on male and female reproduction in plants. Field Crops Research, 182, 30–42.
  6. Austin, C. N., Grove, G. G., Meyers, J. M., & Wilcox, W. F. (2011). Powdery mildew severity as a function of canopy density: Associated impacts on sunlight penetration and spray coverage. American Journal of Enology and Viticulture, 62 (1), 23–31.
  7. Austin, G., & Wilcox, W. (2011). Effects of fruit-zone leaf removal, training systems, and irrigation on the development of grapevine powdery mildew. Am. J. Enol. Vitic., 62 (2).
  8. Alsanius, et al. (2017). Ornamental flowers in new light: Artificial lighting shapes the microbial phyllosphere community structure of greenhouse grown sunflowers (Helianthus annuus L.). Scientia Horticulturae, 216, 234–247.
  9. Elad, et al. (2007). Effect of colored shade nets on pepper powdery mildew (Leveillula taurica). Phytoparasitica, 35 (3), 285–299.
  10. Suthaparan et al (2010). Continuous Lighting Reduces Conidial Production and Germinability in the Rose Powdery Mildew Pathosystem. Plant Disease, 94 (3), 339–344.
  11. Cao, X., Luo, Y., Zhou, Y., Fan, J., Xu, X., West, J. S., … Cheng, D. (2015). Detection of powdery mildew in two winter wheat plant densities and prediction of grain yield using canopy hyperspectral reflectance.PLoS ONE, 10 (3), 1–14.