The natural gardener

garden mildew

Posted on: 4th July, 2017

Category: Home, Garden & Environment

Contributor: West Cork People

Noah Chase studied horticulture at the Eden Project in Cornwall, England. He now co-manages a family run nursery, Deelish Garden Centre in Skibbereen, specialising in rare, unusual and edible plants. His passion is sustainable gardening, useful plants and care of the environment.


As you may already be aware, here at Deelish Garden Centre over the past few years we’ve decided to stop stocking and using synthetic chemical solutions as quick fixes; instead we offer natural alternatives. We’ve been delighted at the feedback from our customers who often offer us their own ideas about natural solutions to natural problems!

In the last article, I discussed using natural alternatives to combat pests and weeds in the garden. This time, I’d like to have a look at some natural fungicides – because warm damp weather is ideal for a wide range of airborne fungal diseases. The most obvious of these are Black Spot on roses and Blight on potatoes and tomatoes. And if we get dry warm weather, Powdery mildew, another air bourn fungus, can also become a problem.

Black Spot

Black Spot is probably the major fungal diseases affecting roses. It’s easily recognised by yellowing leaves with large black spots on them; sometimes even new leaves emerge that are already affected.

Because Black Spot causes the rose to lose its leaves, the plant soon becomes weak, and if it goes unchecked, the ultimate result is that your rose plant will die. This situation can become critical in humid weather and a wet summer, which we often have in West Cork!

In order to get rid of Black Spot you need to reduce the humidity factor of your growing roses, and you can help do this by making sure that when you plant them out initially, they’re not overcrowded. Try not to under plant too closely with annuals and other small plants. If you do decide to under plant, perennials such as Nepeta Walkers Low are an excellent choice, though it’s best to delay adding them until the second or third year after initial planting, as this gives the roses a chance to establish. Ornamental alliums, Basil, and Marigold are some of the more popular companion plants used to repel insect pests.

Make sure that your roses have full sun and don’t water them in the late afternoon or the leaves will remain wet for a long time. Also try to avoid overhead watering of your roses during the summer heat. When the leaves are wet these damp conditions can help spread fungal spores.

Good hygiene is also important to control Black Spot, so rake up any fallen leaves and remove any prunings from the area. When pruning, remove all the old foliage as the spores reside on the dead leaves over winter. If you see any black spot on your roses remove the affected leaves and burn them. Don’t put these into your compost heap, as this can spread Black spot.

A spray composed of one part cow’s milk and two parts water is also recommended to control Black Spot disease. When applied weekly, the solution controls Black Spot, as well as any synthetic fungicide.

Roses are heavy feeders so give them regular doses of fertiliser every six weeks throughout the growing season. This keeps them healthy and therefore makes them resistant to black spot, as well as other rose diseases. Not only that, roses that are well fed and given compost dressings often will out-grow black spot.

Spraying the leaves with a foliar feed is also beneficial. Liquid seaweed fertiliser increases rose health and again helps them resist disease. We find Neudorff Ogranic Rose Feed or Chase SM3 liquid feed both great foliar feeds.

Finally, add some wood ash to your soil in winter and dig it in lightly around your rose plants, as Black Spot often occurs when there is a shortage of potash in the soil.

Potato Blight

We have excellent conditions for growing potatoes here in West Cork; unfortunately our biggest problem is blight.

Traditionally, copper sulphate or bluestone has been used to control potato blight, but some gardeners feel that using large amounts of copper sulphate can have adverse reactions in the body, and it shouldn’t be allowed in organic farming. Luckily we have a natural solution growing here in some of our heavy soils – its Horsetail!

To make a Horsetail concentrate, mix half a cup of dried leaves in five litres of water, bring it to the boil and simmer for half an hour. Cool, strain and bottle. It should keep for a month, and can also be used on other plants; it also has many uses for people including using it as a hair rinse!

For blight control, use one part concentrate to five parts water and spray on foliage once every week or two or daily if it’s raining.

If you can’t find any plants growing or if making it up seems like too much work, we also supply a concentrate here at the garden centre.

As with many garden disease problems, prevention is the best tool. Continually inspect your crop, remove all dead or dying plants and dig up the potatoes. Discard the infected plants far away from your garden. Autumn clean-up is also important. Even without signs of disease — but especially if there are — remove all potatoes and plants from your garden. Blight overwinters in potatoes left in the ground. Don’t add infected plants to your compost. And as with most vegetables, crop rotation is advised, particularly with potatoes.

Tomato blight

Tomatoes are one of the most popular vegetable crops, especially with those fortunate enough to have a plastic tunnel or glasshouse. Blight begins as spots on leaves, which progress to eventually disfigure the tomato fruit before it’s fully formed.

Horsetail, Comfrey and bicarbonate sprays are among the best known broad-spectrum homemade formulas to help plants stay healthy and ward off fungal spores. Use the above Horsetail recipe or alternatively fill a large jar with Comfrey leaves or Horsetail stems, fill with water and leave them to infuse for at least two weeks before straining. The infused herbal water is then diluted by half with fresh water and put into spray bottles. As with other sprays, a small amount of mild liquid soap helps keep the spray on the tomato leaves.

Bicarbonate spray is made from two litres of warm water, one teaspoon baking powder and one or two drops of mild liquid soap. It’s then shaken and decanted into a spray bottle and the spray is applied directly to the foliage of tomato plants.

It’s important to allow adequate spacing between plants. Water only at the base of the plant and early in the day. Long periods of moisture on the foliage encourages blight. Remove the bottom branches of the tomato plant, especially if they come in contact with the soil.

Powdery mildew

If we do happen to get warm, dry weather (fingers crossed), be alert for the development of powdery mildew on crops. Powdery mildew is a common summer problem on many types of vegetables and cut flowers. But most powdery mildews can be avoided or cured with inexpensive, homemade remedies that have been shown to work as well as, or better than, commercial fungicides.

A mixture of milk and water, as mentioned above, works to control powdery mildew; just make sure to spray both sides of the leaves.

A Baking soda-based fungicide is mostly effective as a preventative; offering only minimal benefits after your plants have become infected. If you know which plants are susceptible, spraying them weekly, with the above mentioned bicarbonate recipe, during warm and dry weather can greatly reduce the incidence of powdery mildew in your garden.

Yet another kitchen remedy for powdery mildew is garlic extract, which can be made by blending two bulbs (not cloves!) of fresh garlic in a litre of water with a few drops of liquid soap. The liquid should be strained through cheesecloth to remove solids and then refrigerated. That concentrate should be diluted 1:10 with water before spraying. This concentrate can also be used to deter pests from other edible crops.

I hope that some of the ideas mentioned will help you have a great year in the garden and enjoy finding which natural recipes best suit your plant’s needs. Of course, you’re more than welcome to stop into Deelish to discuss any of these topics with us or to give us your own favourite remedies. Remember gardening doesn’t have to cost the earth!

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