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.
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.
The flat-bodied bronze orange bugs in the photos below are also known as stinkbugs because of the vile odour of the secretion they release when disturbed. These bugs can do a lot of damage to citrus trees as they suck sap from new shoots and flower stems, causing shoots to wither, and crop losses. The secretion is very caustic and can burn foliage. Stinkbugs are commonly seen on citrus trees from winter onwards where a prolonged dry spell has caused water stress.
The young nymphs are green, (about 6 mm long) and can be found in groups on the underside of leaves. As they grow to adults, they can reach 2.5 cm in length, and change in colour through grey-green, to orange or pink-orange, to almost black. Older nymphs and adults are more obvious, clinging in groups to upper surfaces of foliage.
No organic sprays or treatments seem to be effective for these pests. We have found the safest organic way to remove them is to don some goggles or sunglasses and some long rubber gloves (to avoid the secretion burning or staining your skin), and then use a stick to knock them into a fruit tin containing some methylated spirits. It is best to de-bug your tree/s before the weather becomes too warm as, on hot days, the bugs leave the foliage and congregate around the base of the tree. The bugs will die off in hot, dry weather but by then they will have done a lot of damage.
Some gardeners use a vacuum cleaner to remove stinkbugs, but this method is only practical if your cleaner uses disposable paper bags so that you can dispose of the critters in a sealed bag, and if your vacuum cleaner and extension leads are suitable for outdoor use.
In summer, have a look under citrus leaves for stinkbug eggs. The eggs are spherical and fairly large for insect eggs (3 mm), and they are laid in rows. Remove the leaves carrying eggs and dispose of them in a sealed bag.
To prevent future attacks, make sure your citrus trees receive adequate water and are mulched to keep soil moisture more consistent. Also check that soil pH is between 6 and 7, and that your trees have received adequate complete fertiliser for best pest resistance.
The Brisbane Insects website has a lot of photos of stinkbugs (a.k.a. bronze-orange bugs) at all stages of their life cycle, including one of their mating position, and a close-up of exactly which part of their body releases the vile liquid.
Jeanette is having trouble with wireworms in her potato patch and where she has grown yams.
Wireworms are larvae of Click Beetles. The larvae are shiny, brownish cylindrical creatures with 6 legs close to the head. They can grow to about 3.5 cm long, and are fond of eating roots, fleshy stems and tubers.
Wireworms can increase to pest proportions where sufficient crop rotation has not been practiced. The adults shelter in weeds or vegetable litter in and around garden beds.
To get rid of the adults, clear the garden area of weeds, old plants and mulch. You can then get the adults to shelter under a plank or bag laid on a garden bed. Early each morning, collect and destroy the beetles until it is obvious you have numbers under control.
Dig a 3 – 5 cm layer of compost into the top 15 cm of affected beds, or add plenty of organic fertiliser and grow a green manure crop to slash and dig into the topsoil when it is knee high.
Don’t grow yams, potatoes or carrots in that area again for at least four years. Rotate crops, starting with a legume, on a regular basis to prevent a build up of these pests in soil. (See Crop rotation )
Keep weeds under control and apply enough organic fertiliser to keep plants growing vigorously, and they should cease to be such a nuisance for you, Jeanette.
These flirtatious little butterflies can be destructive to stressed Brassica plants. If your plants have been attacked, squash any green caterpillars or feed them to the chooks. They often hide along the mid ribs of leaves making them difficult to see. Check daily for newly laid CWB eggs (bright yellow dots – usually on the backs of leaves), and brush them off. Also check for newly hatched larvae – these appear as fine green threads hiding under the leaves, and can look like leaf veins.
White cabbage moth attack is a sign that either:
1) Your plants could do with more water. Brassicas need thorough, regular watering – not a daily sprinkle. Mulching the bed reduces water loss and encourages horizontal movement of water through soil.
2) You have been a bit mean with the complete fertiliser when preparing the bed. Brassicas prefer a humus rich soil to provide a good supply of fertiliser. If they are not making steady growth, a side dressing of compost (under mulch) or applications of complete fertiliser, applied as a tea, can correct the problem. Or, they are missing some essential trace elements that you can supply with a drink or two of good quality seaweed extract tea (such as Acadian, Natrakelp or Seasol).
3) You have added enough fertiliser but the soil is too acid or alkaline for the plants to absorb what they need for pest resistance. If it is a case of too acid – and this can be remedied with an application of dolomite or agricultural lime. If you suspect acidity, apply a handful per square metre of bed and water it in. Avoid using hydrated lime on beds that contain plants as it can burn plant roots. If your soil is too alkaline, the addition of elemental sulphur will reduce alkalinity.
However, if they are making a total mess of your plants, apply Dipel while waiting for soil conditions to improve. Dipel will kill the caterpillars without killing good insects.
Although bean crops in most areas are close to the end of their productive life (April-May), some gardeners in warm climates are having problems with bean fly.
The best way to avoid bean fly is to have plenty of compost in the bed where you grow your beans so that they will have a full range of nutrients available to them. These pests tend to attack plants that are low in potassium. A drink or two of seaweed extract tea can help bean plants that have been attacked because it has plenty of potassium, as well as trace elements that plants need for good health. But don’t overdo the seaweed as too much can “lock out” some other nutrients.
Also, hill up soil around the main stem of the bean plants to encourage them to send out more roots along the stem above the damaged area just above ground level. Mix some compost through the hilled-up soil. Plants can be saved this way. Also, allow a three year break between growing crops of beans in the same soil.
Bean fly love warm, humid weather. In these conditions put out some yellow sticky paper traps to alert you to their arrival, and hopefully catch some of the adults before they lay eggs. Also look out for fine lines on leaves and stems indicating where the larvae have tunnelled to reach the base of the stem. Stems of affected plants become swollen, cracked and reddish in colour.
Organic sprays are not really effective because the larvae that do the damage are protected by plant cells. A good fertilising and watering routine will help your plants avoid these pests.