This summer has been such an on again – off again season, with spikes of heat interspersed with cool spells, and we even got some decent summer rain. I have been hearing lots of questions about fungal diseases causing problems on vegies and ornamentals. So, what can you do about it?

Firstly, it is important to understand that happy healthy plants growing in good soil are less likely to get pests or diseases than plants which are under stress, whether they are hungry, they have dried out or are suffering from root competition (which basically robs them of moisture and nutrients).

When anything goes wrong in my garden, I try to go through the following five step mental checklist before I do anything else. My garden has some problems with fungal diseases at the moment, including powdery mildew, so let’s apply this checklist and work it through:


  • Step 1. Correctly identify the problem.
  • Step 2. Understand why the problem occurred.
  • Step 3. Decide if I need to act. If I leave it untreated what will happen?
  • Step 4. If it’s an insect pest problem, find out if there are any good bugs to help me clean up the bad bugs. How can I encourage them for the future, and could I possibly buy some to start the process?
  • Step 5. After I have been through all these steps, and I have decided to act, then I choose an organic treatment, and make sure it won’t result in an ecological imbalance that can result in further problems.

Powdery Mildew

So, in the case of powdery mildew which is starting to affect my older zucchini plants, pumpkin and other squash like New Guinea beans, what do I do?

Step 1. Correctly identify the problem.

This common disease affects a range of ornamental and edible plants including roses, old fashioned forms of crepe myrtles and grapes, however it is when it affects cucurbits in the vegie patch such as zucchini, cucumber and pumpkin that you might need to intervene. Starting out as small white spots that appear like fine powder, often just on the new growth, powdery mildew usually spreads to cover large areas of the leaf. Young foliage may become distorted by this fungus.

Step 2. Understand why the problem occurred.

Generally, when plants are strong and healthy and if you have good air circulation and avoid overhead watering, powdery mildew is not a problem. Think back to when you first planted your zucchini, cucumbers, or pumpkins, they don’t usually get affected, even though spring showers and warmth could theoretically create the ideal conditions for it. However, in mid to late summer and autumn, by the time the plants have powered away, grown a lot, been highly productive for several months, they have used up most of the nutrients they were planted with and start to get hungry and tired. This is when powdery mildew may start to appear, especially this year when our summer showers and humidity have created the ideal conditions for it to proliferate.

If you are watering the affected plants overhead and consequently making their foliage wet, make sure that this is done in the morning allowing the leaves to dry out before the evening when wet foliage will create humidity, or try applying the water via drip or soaker hoses at ground level only. I did water the bed with my zucchini and New Guinea beans at night as I wasn’t around during the day, and on those nights, it was cool and the moisture hung around on the foliage. Physical measures that increase air circulation such as making sure that the plants are not overcrowded are also effective.

Step 3. Decide if I need to act.

If I see just an odd, infected leaf, I try to remove it immediately, however if my plant is quite affected, I need to decide if I will intervene.

If I know my plants are hungry, I can choose to try and feed them, but by the time I see the problem, it may be too late to get them to respond in time. I try to make sure I give my susceptible plants a good feed with an organic based fertiliser in late December so by the end of January I have pre-empted their nutrition needs.

If I leave it untreated what will happen? Powdery mildew will eventually cause the plants to stop being productive and they may die, but in the case of these vegies, they are going to die in autumn anyway. So, its whether treating it will make my plants persist and produce for longer.

If the zucchini plants have been highly productive and are on their way out, I may let them go and do a late planting of a few new plants in good, well-prepared soil so that by the time these new plants start cropping I can pull the old ones out.

Usually, I am laid back and am happy to leave it with an understanding of why the problem occurred, planning to do better next year. However, as I open my garden to the public each Easter, sometimes vanity gets the better of me and I decide I need to treat the mildew so my plants good when visitors come.

Step 4. Only applicable to insect pests

Step 5. Choose an organic treatment

Many gardeners have used a homemade milk spray as a fungicide (1 part milk to ten parts water) which can work. Scientists think that milk proteins react with sunlight to create an antiseptic ‘effect’ however they are effectively only when applied in bright sunlight and if not applied correctly it can cause its own problems such as lactose residue and sooty mould.

However, I prefer to use eco-fungicide, an effective organic fungicide based on food grade potassium bicarbonate. It is effective under low light levels and it creates a more alkaline environment on the leaf surface which disrupts the germination process of the fungal spores and damages the mature spores’ cell walls. To control an infection, you need to catch the disease quickly and while it does kills the fungus in minutes, for it to work properly, good coverage is essential. There is no withholding period which means you can spray the plant and still harvest your produce the same day. Finally, it is also safe for beneficial insects and soil microbes.

Whenever something goes wrong in your garden, take a few moments to go through this process and hopefully it will take the pressure off thinking you need to treat everything.