Powdery mildew

pwdrymildw 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.

This entry was posted in Pest-free Gardening and tagged , , , , , , , , , , , , , , , , , , , , . Bookmark the permalink.

11 Responses to Powdery mildew

  1. Great post. I’ve read lots of other posts about how to spray to get rid of powdery mildew, but have never read about the temperature or the link to insufficient fertilizer. Thanks!

  2. adele Taylor says:

    My snow peas got affected with mildew … What I’m not sure about is whether to chuck them out in the rubbish bin or is it OK to put in compost bin?

    Adele, as the fungi that causes this problem is spread by air currents, you can chop them up, wet them down, and put them in the compost bin if it is a moderate infection. Then put a layer of manure over them, and the good bacteria will break them down fairly quickly. If the plants are badly affected, and you don’t have an active compost heap, then bag and bin them. – Lyn

  3. greg says:

    What about milk spray? Recommended often, and a study by a university in Adelaide found it not only effective, but determined the mechanism at work. Something to do with free radicals and the reproductive structures of the fungus. Spray in full sun!
    Greg, that is a very interesting explanation for how milk spray works. I understood that it (and baking soda spray) worked by changing the pH of the leaf so that the fungus can’t survive. However, I’ve found a 10 percent milk spray more effective for downy mildew and black spot.
    Powdery mildew is produced by a different group of fungi and its cause is insufficient potassium – either through inadequate fertiliser or inadequate watering when the plants can’t absorb the potassium from dry soil. Organic seaweed extract contains potassium and a range of trace elements that plants need to resist disease. Chamomile tea is an organic fungicide that works on a range of fungi.
    Milk spray makes leaf surfaces more alkaline, whereas wettable sulphur (a common treatment for powdery mildew) has an acidifying effect.
    Sulphur can’t be used in hot weather or when plants are short of water. – Lyn

  4. Chris says:

    When making the chamomile tea spray, why does it need to be diluted? Will it burn the foliage if used undiluted?

    Chris, undiluted chamomile tea has a silky feel (which is why it is an excellent hair rinse, too). I don’t know about it burning leaves but perhaps what gives it the silky feel could clog pores on leaves. I know it needs to be diluted for seedlings but if you want to treat zucchini or larger plants, you could try it undiluted on a couple of leaves – observe the results – and share your findings with the rest of us. That’s how most good gardening tips come into common practice. – Lyn

  5. Annie Ludford says:

    How do I tell the difference between powdery and downy mildews? My zuccinis are pretty bad at the moment.
    Annie, downy mildews are furry, and powdery mildews look as though the leaves have been roughly dusted with a fine white powder. Downy mildews usually only affect stressed zucchini in very wet weather, and powdery mildews are more common on zucchini in warm, dry conditions. As you have not said where you live, it is difficult to make a judgment on which one would be affecting your plants.
    Downy mildews respond to a 10% milk solution, which makes the leaf surface more alkaline and the fungus can’t survive. The chemical treatment for powdery mildew is sulphur (which is acidifying) and the milk treatment is not likely to be effective for this fungus group. Plants affected by either disease will benefit from the application of fertiliser that includes potassium. – Lyn

  6. Brenna says:

    is it OK to harvest pea seeds from plants that are going mildewy? thanks… it’s hard to find an answer to this!
    Mildew on peas is a sign the plants have exhausted their fertiliser. If the mildew is just beginning and only starting to affect the outside of the pods, check the peas inside the pods and if they are unaffected, harvest all the peas and freeze any excess. If the pods have become soft you would probably contaminate the peas when shelling them, and you will probably have to ditch them. – Lyn

  7. Kate Milne says:

    Is the powdery mildew poisonous? My 2yo daughter preys upon the peas mercilessly and she has just become sick and they are heavily infected with the mildew?

    Inhaling spores from moulds and mildews are not good for anyone’s health, and children are more susceptible to problems because their immune systems are not fully developed.
    It could be that she has inhaled spores or it may be something entirely different. I’d have your daughter checked by a doctor as soon as possible, and remove the peas if you can’t control the mildew or keep her away from them. – Lyn

  8. mel says:

    I have powdery mildew on my pea plants that are just flowering and have only been able to find the seaweed extract and not the chamomile treatment how much should I dilute it to spray my plants? the bottle says 4 tablespoons per gallon / 15mL per litre to water can I spray it with the same?

    Hi Mel, if the bottle says the rate for that seaweed solution is 15 ml (3 tea spoons to a litre of water), spray it at the recommended rate. As powdery mildew only attacks stressed plants,as the plants are just flowering I would also make sure the the pea plants have adequate water and I’d also give them a light application of complete organic fertiliser and a light dusting of dolomite or garden lime to help them produce a good crop. The other cause of stress to pea plants is warm temperatures – they grow best in the cooler months. If you live in an area where the weather is getting hot, they may not improve.
    The “chamomile treatment’ is just diluted chamomile tea. Chamomile tea bags can be found at most supermarkets and health food stores. – Lyn

  9. adri says:

    Hey, funnily enough, my chamomile plant got what I’ve figured out to be a powdery white mildew, what do you recommend I do?

    Oh, dear, never heard of that before. It sounds as though your chamomile is short of the nutrients it needs for immunity. I’ve found that plants attract powdery mildew when they have run out of fertiliser. Water in some complete organic fertiliser. – Lyn

  10. ROSS> says:

    HI. MY PEAS, especially my snow peas, go black from the ground up, then gets black spots on the leaves, until it slowly kills them. Meantime they produce beaut peas but not for long. Heeeelp.
    Hi, Ross. Your pea plants sound as though they are affected by a Mycosphaerella fungus. It can be caused by sowing infected seed or spores blown by wind from affected plants, possibly eucalypts.
    Usually affected pea plants are removed and destroyed but, as your plants are still producing, you can try feeding them with an organic complete fertiliser and a drink of seaweed extract tea. Plants usually succumb to diseases when they do not have enough nutrients to keep their immune system strong. The potassium in seaweed helps strengthen cell walls of plants so that fungi find it more difficult to establish. Remember to water your plants regularly as they can only absorb the nutrients in water-soluble form. – Lyn

  11. Genna says:

    Novice question – but are you referring to organic chamomile tea that can be brought from any grocery shop? And with the milk dilution, 1 parts milk to 9 parts water?
    Thank you

    We are all novices at some time, Genna. Yes, if I don’t have any chamomile growing, I use bought chamomile tea bags for POWDERY mildew spray. The 1 part milk to 9 parts water spray is for DOWNY mildew. Full fat milk is best – it helps it stick to the leaves. – Lyn

Leave a Reply

Your email address will not be published. Required fields are marked *