In a previous blog post we discussed plant pathology in a broader sense, and while there are way more devastating plant pathogens powdery mildew is one of the most common ones effecting a wide variety of plants. While it isn't typically fatal it can make a substantial impact on yields. It is easily identifiable, first as erratic white spots on the leaf. Over time it will cover the whole leaf turning it white, and since direct sunlight will inhibit growth it's more likely to show up in shaded areas or on the underside of leaves.
The are obligate parasites, meaning that they can only survive on living material. This makes studying them in the lab more challenging as they can't survive on artificial media. While dead plant material won't contain any living pathogen, it may still harbor spores.
There are Many Different Powdery Mildews
With most diseases we think of them as only one species or genus. For example Botrytis (grey mould) infects a number of different species of plants but the different kinds of Botrytis are all closely related. Powdery mildew is different in that there are a large number of distantly related fungi and they are host specific.
A few examples of this:
Roses: Sphaerotheca pannosa var. rosae
Cereal grains (including wheat and barley): Blumeria graminis
Hops and Cannabis: Sphaerotheca macularis
Apples and Pears: Podosphaera leucotricha
The Circle of Life
- Infection starts when a spore lands on the leaf of a plant which is susceptible to infection, and if conditions are right it will germinate.
- A rootlike structure will begin to grow out and eventually force itself into the space between the leaf cells.
- From there it will begin to parasitize the cells inside of the leaf, deriving nutrients from them. Once the energy source is established threadlike structures called mycelium will spread across the leaf, penetrating in multiple locations.
- The fungus now start to build tower like structures off of the leaf. Each of the cells on this structure will turn into a new spore. These get sloughed off and are carried by the wind to carry out new infections.
The Disease is Dependent on Temperature and Humidity
The optimum temperatures of these fungi are generally in the low 20's Celsius, and they are inhibited when the temperature goes over 30 Celsius. For this reason infections are less likely during the hottest summer months and more likely early or late in the season. Some types of powdery mildew are not able to overwinter in harsh winter conditions, this means that in those areas there will be a lot fewer infections than in areas with milder winters.
Likewise it prefers high humidity levels. In greenhouses and indoors keeping a low humidity level is crucial in keeping outbreaks from getting started. Like most pests preventing the problem is a lot easier than treating it once it has been established.
An Ounce of Prevention...
Once an infection has been established it is impossible to eradicate it completely, the best that a grower can hope for is to keep it under control long enough to pull off a harvest. The better way of doing things is to keep the disease from ever starting in the first place, here are a things to keep in mind.
- As mentioned in the previous section, try to keep the humidity low. While high temperatures will slow down powdery mildew there's too great a risk of problems from other types of pathogens by running too hot.
- Dead plant material can harbor powdery mildew and other pathogens. Be sure that dead leaves are disposed of and the growing area kept as clean as possible.
- Keep a close eye on your plants for signs of infection, this doesn't just apply to powdery mildew but for all pests. This is especially important when conditions are favorable for an outbreak. If an infection is starting then trim off the leaves that have signs of infection and if space permits quarantine the plant. If the infection is more severe but confined to a small number of plants consider destroying them before it endangers the rest of them.
- Choose a resistant variety. There are a lot of variables to consider when selecting a variety that will work best for the situation, however disease resistance should be taken into account. A variety can theoretically yield high with a high quality product but if that same variety is susceptible to disease then those advantages are decreased as the infection will lower both the quality and the yield. Consider something with average yield and quality and superior resistance instead, especially if there's been a history of disease outbreaks in the area.
*Did you know?*
Powdery mildew is a huge problem in cucurbits which includes squashes, cucumbers, gourds, and melons. There is a lot of research underway to breed genetic resistant varieties. The first resistant pumpkin varieties were produced at Cornell University in 1998, Magic Lantern and Merlin. Plant breeders selected pumpkin varieties that produced a high quality fruit but were susceptible to powdery mildew and crossed them with a wild cucumber variety which was very genetically resistant. After a few generations of selective breeding varieties with both resistance and quality fruit were produced.
My Plants Have Powdery Mildew, Now What?
All the preventative steps have been taken but it's been identified and is spreading. Infected leaves have been trimmed and humidity has been lowered but the crop is still at risk. It's unlikely even the strongest chemicals can eradicate it, but it is possible to control it. There are a number of options, from biological to home remedies, to synthetic chemicals. With any kind of spray, be sure to apply to all of the leaves not just the ones with obvious infections. There could very well be infections there which can not be seen yet, these are the ones that we want to get rid of.
- Require high humidity in order for growth, which are the same conditions as the pathogen.
- Have a significant lag time as the population grows large enough and establishes itself in order to be effective and so is more of a preventive measure.
- The microbes take up space on the leaf surface offering competition, some can also produce chemicals to inhibit pathogens.
- Examples include Bacillus sp., Trichoderma, and Gliocladium.
- Generally regarded as very safe.
"Home Remedies" and Over the Counter Fungicides
- Oil based products which literally smother the pathogen. They are usually emulsified with some type of soap in order to dissolve them in water.
- pH adjusters such as baking soda or citric acid act by changing conditions to acidic or alkaline which inhibit the pathogen by changing its growing environment.
- These have to come into direct contact with the powdery mildew to be effective and quickly become inactive.
- Safe and inexpensive.
- Overall performance isn't very good and will have to be used continuously.
Synthetic Chemical Pesticides
- Most heavily regulated and expensive. Often times only large volumes will be available for sale making them impracticable for small growers.
- Generally the most effective in controlling powdery mildew.
- Mostly safe for humans and animals but can be dangerous to aquatic ecosystems.
- Work best when applied before the disease becomes a problem.
*Did you know?*
Copper Sulfate is widely used as a fungicide in organic agriculture for well over a century. And while it is found naturally it is not a safe chemical. With an LD50 of 30 mg/kg, only 3 grams (less than 1 tsp) would be a potentially fatal dose for most people. It is also extremely toxic to aquatic ecosystems. Some organic wineries are switching to conventional methods as their soil has become so saturated wiht Copper Sulfate over the years that is starting to effect the soil health.