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Summer pest and disease watch

28 January 2022
Passionfruit leaf spot
Passionfruit leaf spot

What to look out for at this time of year and how best to take action. Words & Photos Andrew Maloy.


These are the larvae of moths and butterflies that often feed from the undersides of leaves, out of sight and protected from the elements. Holes appearing in leaves are a sure giveaway. You can deal to them digitally (pick ’em off and squash ’em) if you have the time and patience.

Nowadays there are a couple of very useful products that make life unpleasant for caterpillars, both derived from naturally occurring soil organisms that have no adverse effect on humans so are suitable for use on edible plants. Bacillus thuringiensis var. kurstaki (Btk) is the active ingredient in Kiwicare Organic Caterpillar Bio Control and some other brands. You spray it on foliage and caterpillars ingest it as they feed, causing gut paralysis and death soon after. Yates Success Ultra contains the active ingredient spinetoram, which not only controls caterpillars but some other summer insect pests too, including thrips and tomato potato psyllid. It has the added benefit of being translaminar, moving into the leaf within six hours of application, making it resistant to breakdown by rain and sunlight.

Guava moth

The larvae of this moth damage fruit on a wide range of plants, including some natives. Female moths lay eggs on the outside of immature fruit, and larvae bore into the flesh where they’re protected from insecticides.

The most effective control is to cover whole trees with netting to prevent female moths getting to the fruit. The moths are small, so nets must have fine mesh and need to be put in place after flowering has finished, so pollination can occur but only while fruitlets are still small.

Guava moth pheromone traps are also available; you need to closely follow the label recommendations and hang them in susceptible fruit trees as soon as fruit begins to form. They catch lots of guava moths and help protect fruit, but you need quite a few traps, one in each medium-sized tree, which can be rather costly.

Guava moth damage to oranges and feijoas
Guava moth damage to oranges and feijoas

Tomato potato psyllid

Thriving in hot summer weather, the adult is a tiny flying insect and both it and the nymphs (young) suck sap from leaves and stems, spreading a bacterial disease that causes stunted growth and poor yields. Check your tomato and potato plants regularly; if you spot the problem early enough, spraying with Yates Success Ultra will help, or you can use neem oil – make sure to get thorough coverage of all foliage.

Mites & thrips

Hot, dry weather is bliss for these tiny sap-suckers, which between them infest a wide range of garden plants. There are several species of each, some more specific to certain plants than others. Their damage symptoms can be somewhat similar, so the only way to tell for sure what you have is to carefully inspect the undersides of leaves. Thrips are easier to identify as they leave shiny black blobs of excretions.

The most effective way to control most of them in the home garden is with oil sprays, such as Kiwicare Organic Super Spraying Oil or Yates Conqueror Oil. These are non-toxic and safe to use on edibles, though make sure to follow the label recommendations when it comes to the dilution rate as too strong a mix can damage foliage and young fruit. Spray in the cool of the evening or on a cloudy day, as applying oils in hot sunshine can also lead to damage. Make sure to get thorough coverage of upper and lower leaf surfaces and stems as the oil works by smothering the insects and eggs. Yates Mavrik also controls mites and other pests, but be aware of the necessary withholding period before using it on edibles.

Greenhouse whitefly & citrus whitefly – the terrible twins

Both these species of whitefly are common pests in summer in some regions, when populations boom. Greenhouse whitefly (so called because it can be a serious pest in greenhouses worldwide) is common here on outdoor crops of tomatoes, courgettes and many other vegetables and ornamentals. Growing nasturtiums among susceptible plants is an effective way of reducing whitefly infestations, and you can add the nasturtium flowers and leaves to a fresh salad. Alternatively, spray regularly under the leaves in the cool of the evening with a pyrethrum or neem oil product, or Yates Nature’s Way Vegie Insect Spray Natrasoap.

Citrus whitefly, as its name suggests, is found only on citrus species. Control it by spraying with insecticidal oils as described earlier for mites and thrips. They’re safe to use even when the tree is carrying fruit and will also control scale.

Leaf spots

Caused by a range of different fungi, leaf spots can affect some vegetables, fruiting plants, ornamentals and native plants, damaging leaves, flowers and fruit. Common leaf spot diseases include apple black spot, rose black spot, passion fruit leaf spot and renga renga lily leaf spot. Leaf spots are generally more prevalent in damp conditions, so growing susceptible plants where they’re exposed to as much sun and fresh air as possible helps foliage dry quickly after rain and reduces the chances of severe infection. Fungicides useful for leaf spot control include copper-based and sulphur-based ones that are safe to use on edibles, while some that control leaf spots on ornamentals like roses should not be used on edibles, so read and follow the label recommendations.

Powdery mildew

The fungal spores germinate in warm weather, so this disease is often at its worst from mid- to late-summer. Again, a range of fungi species is involved on a wide range of plants, including begonias, cucurbits (pumpkins, courgettes, cucumbers and so on), dahlias, grapes, pansies and peas, including sweet peas.

The first sign of infection is small whitish-grey powdery spots, which then spread over the leaves. With some plants, such as tamarillo, infected leaves may drop off. Some plants are bred to have some resistance to powdery mildew, but it can still be a problem in hot weather. You can’t control the weather, so you may choose to spray susceptible plants before the fungus gets well established and follow up at regular intervals.

There are several fungicides from which to choose, including Yates Nature’s Way Fungus Spray and Kiwicare Organic Super Sulphur. Note that copper-based fungicides alone are not effective against powdery mildew. You can make a home-made organic spray by mixing 1 teaspoonful of baking soda in 1 litre of water and adding 2ml of dishwashing liquid. Apply this at weekly intervals for best effect. Or you can use the ready-made option, sold as Eco-Fungicide, containing potassium bicarbonate as the active ingredient. Spraying plants regularly with seaweed-based foliar fertiliser may also reduce the severity of powdery mildew attack, and oil-based sprays are reported to have a similar effect by reducing the ability of fungal spores to germinate on the leaf surface.

Remember, always take time to read product labels and make sure to follow the manufacturer’s recommendations.

Whitefly on courgette plant.
Whitefly on courgette plant.
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