A reader has asked about powdery mildew on zucchini plants and fungus-eating ladybirds: Hi. Wonder if you can sort this. 1. Most fungi need moisture and organic material. This seems to be supported by my zucchinis which seem to get worse powdery mildew when I get water on the leaves. I have read that they like dry weather. Is there evidence for either opinion? 2. Some people say that the ladybirds that feed on this mildew spread it by carrying spores, others reckon they are a controller, eating the fungus down. What is the evidence please for either of these? Many thanks, Barb
Powdery mildew is likely to occur on stressed plants in humid weather when temperatures are between 11-28° C. and, once established will continue to affect the plants even if weather becomes dry. Avoid wetting leaves whenever possible Barb. However, because they like low-humidity weather, it doesn’t mean that they are drought tolerant. Zucchini and some other members of the cucurbit family (melons and squash) produce a lot of foliage and need plenty of water and fertiliser. An efficient way to water this group of plants without wetting the leaves is to put a large drink container (with the base and cap removed) neck downwards near the roots so that all the water goes directly to the root area where it is needed, (see photo). Keep topping up the container until it empties slowly.
The yellowish ladybirds with 26 or 28 spots are the only pests of the ladybird family. They eat the leaves of stressed plants of cucurbits. The beneficial fungus-eating ladybird and larvae can be clearly distinguished from the pest in the photos below. From the far left is the ‘Fungus-eating ladybird’, then the leaf-eating ’26 spot Ladybird’ that damages plants. Next is the larva of the ‘Fungus-eating Ladybird’, which also eats fungus and, last of all is the prickly larva of the ’26 spot Ladybird.
Rather than blame the fungus-eating ladybird for spreading the disease, gardeners should check that their plants have sufficient water and nutrients to avoid stress, and the soil pH is suitable for them to absorb what they need for healthy, disease-resistant growth. Also see Powdery Mildew for treatment of this disease.
Many parts of Australia are experiencing flooding, while West Australia is experiencing extremely hot weather with bushfires in the south, so looking after the garden is the least of their problems. My sincere sympathy to all those affected by these extreme weather events, many of whom are facing heart-breaking work to rebuild their lives.
For those of us who are fortunate enough to still have gardens, the prolonged wet conditions provide the perfect conditions for downy mildew to flourish. A set of three conditions (10:10:24) is necessary for downy mildew to establish – a minimum of 10 mm of rain, a temperature higher than 10 degrees Celsius, and foliage that stays wet for more than 24 hours. It is also more likely to affect plants that are stressed for some reason, and where there is poor air circulation. This group of fungal diseases produce pale green or yellow spots on the upper side of leaves, and white-grey furry patches on the underside of leaves of a wide range of plants, including cucurbits, the cabbage family, lettuce, onions, peas and grapes (starts as oily spots). Different species of the fungi infect different varieties of plants, so that downy mildew on your cauliflowers does not mean that other vegetables in your garden will be affected.
The best thing you can do is remove badly damaged foliage and dispose of it in a sealed plastic bag – compost it! Then give soil around affected plants a drink of seaweed extract tea at the strength advised on the label. A good potassium content in seaweeds strengthens plants cell walls helping plants to build resistance to diseases.
Spray remaining leaves with 100 mls milk in 900 mls water (to make 1 litre of spray), and add a good pinch of bi-carbonate of soda (baking soda) for every litre of spray. Full cream milk works best because the fat content helps the spray stick to the leaves, and full cream, organic milk is even better (according to some gardening gurus) – if you can spare it. Milk and bi-carb are not fungicides, but they produce unsuitable conditions on leaf surfaces for the fungi’s survival. Spray leaves early in the day, and repeat every four or five days until mildew clears.
Downy mildew tends to disappear as weather becomes drier because it needs constant humidity. However, powdery mildew needs high humidity, but not wet weather, to establish and will continue to flourish after weather becomes drier. Powdery mildew is caused by an entirely different genus of fungus and treatment for that disease can be found here.
P.S. When spraying – be careful to avoid these tiny fellows below – the adult and larva of the Fungus-eating Ladybird.
Temperatures between 11-28° C. and excess humidity (without rain) can provide suitable conditions for powdery mildew spores to become active, especially on plants have been affected by drought, or are under-fertilised.
Powdery mildew spores are carried by air and, once active, will continue to spread in dry conditions. This fungal problem affects a wide range of fruit, vegetable and ornamental plants. In most plants, it shows as a dusting of grey-white powder on foliage, and distortion or puckering of new leaves. The infection often begins on the underside of leaves. On mangoes, fruit develops brown to purple patches, and grey patches on papaws. Apples develop light lines across the surface of fruit.
Preventative spraying with wettable sulphur is not recommended because sulphur is damaging to beneficial insects that keep pests under control, and a pest outbreak will often occur after spraying or dusting with sulphur. Sulphur will also damage plants if applied to plants that are short of water, or when temperatures are above 30° C.
Powdery mildews are usually caused by Oidiumspp. fungi, and can be controlled by organic powdery mildew treatment or applications of German chamomile tea. For each 500 ml of spray required, steep one teabag in a cup of boiling water for 15 minutes, then dilute to 500 ml with cold water. Remove and destroy severely affected leaves, then spray the rest of foliage early in day so that leaves have time to dry before nightfall. Don’t forget to spray both sides of leaves.
Powdery mildew is common where plants are deficient in potassium and some trace elements, as when the plants have exhausted their supply of fertiliser, or when they cannot absorb nutrients because soil is too dry. Seaweed extract is rich in both potassium and a range of trace elements (including sulphur), and spraying foliage with seaweed tea can be effective against powdery mildew, not because it kills the fungi, but because it quickly provides the nutrients plants require to resist these fungi.
To avoid this problem in future, ensure that fruits and vegetables have adequate complete fertiliser to last them through harvesting, including an annual application of seaweed extract tea to soil around plants. also ensure that they receive adequate water for steady growth but avoid overhead watering. It is difficult for some gardeners to understand that good cultivation practices can prevent pest and disease problems but it is true. The pea plants in the photo only developed powdery mildew after I had collected pods for seed and I had ceased to water them.