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.
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