Preventing and Overcoming Mildew

Yesterday my peas and tomato plants looked vigorous and healthy.  Today mildew arrived seemingly out of nowhere and my plants suddenly look very ill. Where did the mildew come from? Could I have prevented its arrival? And what can I do to get rid of it? This blog post tries to answer all these questions.

A tomato leaf affected by Powdery Mildew

What is Mildew and how did it arrive?

Even in the cleanest garden an outbreak of mildew can be inevitable. If the weather is warm and humid for a few days, and it remains humid at night, mildew that arrived in your garden via wind, windswept rain, a bee, an aphid, or a bird may get a foothold, no matter what you do.

In Tasmania we get Powdery Mildew and Downy Mildew.  Powdery Mildew is most prevalent and discussed first.  For more on Downy Mildew see later in this post.

Powdery mildew is a powdery-like coating of fungal spores that forms on both upper and under sides of leaves. The powdery coating is often white, but it can also be yellow or black. Once it has taken hold it can continue to grow even in dry conditions. Leaves can distort and fruit may fall off or not fully develop.

Food-plants affected are apple, quince, pear, peach, barley, strawberry, cucurbits (pumpkin, zucchini, cucumber etc.), grape, tomato, brassica, pea, bean, onion and paw paw.

Now, this might lead you to think that there is just one type of Powdery Mildew and that, if you have Powdery Mildew on your peas, it might spread to your tomatoes. 

The reality is that there are many species of fungi that cause Powdery Mildew and most of them are specific to either one host species or a narrow range of closely related hosts.  In other words, Powdery Mildew on your peas might spread to your beans, but it will not affect your tomatoes and vice versa.

If your peas and tomatoes get Powdery Mildew at roundabout the same time then the likely cause is the weather.

Peas badly affected by Powdery Mildew

Ten ways to deal with mildew

The following food-gardening practices are recommended for all gardening situations, but they are particularly useful in the prevention of Powdery Mildew:

• Keep a  clean garden: in autumn make sure that there are as few places as possible for Powdery Mildew to survive the winter.  Tasmanian winters have become milder.  Many of us will get little or no frost in their garden in winter at all.  This is great because we can grow more crops over winter.  Powdery Mildew likes it too, because now, in many cases it is able to survive our winters in nooks and crannies of old leaves and branches.  In autumn carefully collect all dying and dead vegetation and don't put it on the compost heap, because, unless composting takes place at high temperatures (over winter this is less likely), Powdery Mildew on diseased leaves and branches that are put on the compost heap can be the source of next season's infection.

• Grow cool-weather vegetables when it is cool: Some vegetables (peas are a good example) are really not meant to be grown in warm humid conditions.  You can take the risk and sow peas in the middle of summer, but in Tasmania your chances of a good crop and no Powdery Mildew will be much higher if you sow your peas during winter. 

• Use varieties that have resistance to Powdery Mildew.  Growing resistant varieties of cucumber, squash and melon can make a really big difference.  Tomato variety Sweet Bite is said to have excellent disease resistance.

• Provide good ventilation around your plants by not planting them too densely and by removing some lower leaves. In addition, in a hothouse make sure there is always ventilation.

• Don't water in the evening.  Mildew thrives when water does not evaporate, but sits on leaves.  Water early in the morning instead.  Water evenly around plants and regularly, and mulch to keep soils moist.

• Water plants from below, so you avoid making leaves wet. Use a watering can, a hose with a soft spray or a drip system.

• Make plants strong by spraying them once a week with diluted seaweed concentrate (30 ml of concentrate per 9 litres of water).  

• Cut off mildew-affected leaves when you see the first affected leaves with secateurs that were cleaned with bleach or (better still) alcohol (70 - 100%) and put affected leaves directly into a moist plastic bag that you put in the rubbish bin. Don't put the leaves in the compost heap.

• Separate affected plants: make sure affected leaves or plants do not come into contact with healthy ones. In some cases this can simply mean pruning them so they no longer touch. Try to make sure you do not transfer the fungus to other plants on your hands, tools or clothing.

• Use preventive sprays: when warmer humid weather is forecast make leave surfaces as unattractive as possible for any arriving mildew spores by spraying them with milk spray or bicarbonate of soda spray (see below).  Both sprays are commonly used and allowed in certified organic agriculture.  Use these sprays once a week. Spray in the morning, not in the evening.

Three easy-to-make sprays against mildew

Milk spray is made by combining water and full cream milk in the ratio 9 : 1. If Powdery Mildew is present, milk spray kills it, but interestingly, it needs sun light (NOT bright sun shine) to do this, so don't spray milk spray at dusk or after dark (for more info about this see for instance here). Give both the leaves and stems a good even coverage.

Bicarbonate of Soda spray is an effective preventive fungicide against Powdery Mildew, Rust and Black Spot. Jerry Coleby-Williams (ABC Gardening Australia) uses the following recipe: put a drop of vegetable oil and a drop of dish wash liquid in 2 litres of water and add 4 teaspoons of bicarbonate of soda (for more info about this recipe go here). Bicarbonate of Soda is not organic, but used in small quantities it is harmless.

Garlic spray is also effective against Powdery Mildew. Garlic spray is effective against small sucking insects such as aphids, larvae, slugs and snails. Soft-bodied insects can be killed outright by a strong garlic spray. Garlic is also known for its anti-fungal and anti-bacterial properties. I have not used it myself against Powdery Mildew. Here is a recipe:

1. Chop or crush around 100 grams of garlic
2. Mix with 40 mls of mineral oil (colourless odourless non-vegetable oil), 500 mls of water and 25 grams of soap
3. Mix well, then stand for at least 24 hours
4. Filter and dilute at 15 mls per 1 Litre of water

Commercial sprays against mildew 

There are chemical preparations that promise to completely kill Powdery Mildew. These products would all have been thoroughly tested prior to registration. I don't see any need for them as the sprays described above work in most situations and are are low-cost and organic. If you decide to use them, please adhere to the withholding periods (minimum time between last application and use in order to minimise residues) mentioned on them.

What is Downy Mildew?

The second less common form of mildew is Downy Mildew. Symptoms include large, angular or blocky yellow areas on the upper surface of leaves. See the photos below. The underside of infected leaves often appears water-soaked. When looked at closely, a grey-brown/purple-brown mould becomes apparent. 

This photo shows a cabbage plant affected by Downy Mildew

Downy Mildew in a Cucurbit leaf

Food plants affected are mainly potato, grape, brassica, hop, onions, lettuce and cucurbits.  Downy Mildew comes in many crop-specific species, so Downy Mildew affecting your potatoes will not affect your brassicas.

Downy Mildew generally appears in late summer.  However, in 2014 in Tasmania it was reported to take hold of hop crops in November (late Spring) when there was a period of warm humid weather.

To avoid Downy Mildew it is recommended that you choose early season varieties of likely-to-be-affected crops (see above).  Prevention and treatment of Downy Mildew is the same as for Powdery Mildew.

Transferring mildew via seed

Every year I collect pea and bean seeds from my own bushes at the end of the season and used them to sow the following year.  My crops have been fine, even plentiful, but every year I do get Powdery Mildew in my peas and beans after most of the crop has been harvested. 

One of my questions when researching this subject was: am I in fact responsible for transferring Powdery Mildew from one season to the next by saving pea and bean seeds of plants that were affected by Powdery Mildew?

Margaret W, a Food Garden Group member with professional knowledge on the subject, provided the following answer: 

I would keep on saving my own seed, Max.  Downy Mildew is carried over on stubble and in the soil.  While Powdery Mildew can be found on seed there is no evidence of its transmission to seedlings.  Your words about successful pea and bean crops bear this out. If seed transmission was a problem, you would not have got much of a crop.

Thank you!

Tank you, Food Garden Group member and entomologist Margaret W, for checking the accuracy of this blog post and supplying additional information.

Wishing you many mildew-free crops!

Max Bee

Biosecurity Note

Tasmania has a number of species of Powdery and Downy Mildew that attack a range of plant species, but there are many that do not yet occur in Tasmania.  If you see a type of Powdery or Downy Mildew on a plant that you have not seen before, you might want to contact the Emergency Plant Pest Hotline on 1800 084 881 (all hours).

For more information on what to do if you think you have found an exotic plant pest see the biosecurity fact sheet available from http://dpipwe.tas.gov.au/Documents/Emergency_PlantPests.pdf