Powdery Mildew

What causes the white, cottony growth on the top side of many plants including beans, cucumbers, squash, papaya and roses? This is a fungus, called powdery mildew (PM).  There are many different kinds of powdery mildew fungi, but they are rather host specific. This means that one specific mildew fungus will only infect those plants in a particular genus or family; so you don’t have to worry about the PM on the beans attacking the papaya or the PM on your mango attacking the tomatoes.  Powdery mildew fungi like temperatures between 60 and 80 degrees F.  Infected leaves often turn yellow to brown, and may shrivel up and die, making the plant unproductive.

Control –there are a number of relatively safe materials to use against this fungus: wettable sulfur, horticultural oils and Kaligreen (potassium bicarbonate).  Cucurbits (melon, squash cucumbers) can be sensitive to sulfur.  Do not apply when the temperature is near or over 90 degrees and do not apply within 2 weeks of an oil spray.  Apply these materials in the earliest stages of disease development for best results.  Before spraying, it is helpful to remove the leaves that are heavily infested.  Do not dispose of them on the ground since they are loaded with fungal spores. 

A recipe for the control of PM from the University of Hawaii CTAHR calls for one tablespoon of baking soda mixed with 2 ½ tablespoons of vegetable oil per gallon of water; spray every 7-10 days. 

In addition, there are two biological fungicides which work against the powdery mildew fungus.  One is a parasitic fungus.  The other, Serenade, which is registered in Hawaii, is a bacterium that helps prevent the powdery mildew from infecting the plant. 

Powdery mildews can and do attack healthy plants, but older plants that are less vigorous and plants that are stressed are more susceptible to infection. 

Cycad Scale

The large infestation of white bugs on cycads indicates a pest called Cycad Scale.  These white scale insects are found on the underside of the leaves, sucking the tree sap and causing the leaves to turn yellow, then brown and eventually die. Chemical control is difficult for two reasons: the females’ hard, waxy covering and the difficulty to thoroughly spray the undersides of leaves. With heavy infestations, it is best to prune all leaves. A tiny black lady beetle, present on the Island of Hawaii (the Big Island), is highly effective in controlling the cycad scale; there is good biological control. If these beetles are found on a cycad in your area, trim off some of its leaves (with the predatory beetle on them) and place these leaves on your scale infested plants to help the predator migrate. 

Integrated Pest Management - A New Approach

Other than chemical control of pests, what are other ways to control insects or diseases? 

There are four basic approaches to pest control other than chemical.                      1. Cultural control means growing healthy plants since stressed plants are more susceptible to pests.  Plants that are water stressed, i.e., too much or too little water, or plants that are starving for nutrients, will be more susceptible to disease and insect attack.  As an example, some pine trees give off a chemical when they are water stressed. A beetle is attracted to this chemical and will subsequently lay eggs in the tree bark of the tree and cause damage.

Sanitation is also a part of cultural control.  Often it is helpful to simply remove diseased leaves and discard them, or remove the diseased plant among healthy ones.

2. Physical and mechanical control -- this may include using a flyswatter, screening out pests, washing insects off a plant with a hose, or skirt pruning to keep some ground pests, such as snails, out of trees.  In this case, once the trees are skirted, the trunk becomes the only access way into the tree. Wrapping a copper band around the trunk will repel snails from climbing into the tree.

3. Biological control – a.) the use of parasites, such as tiny wasps which lays their eggs inside other insects, and b.) predators like ladybird beetles and praying mantis.  Certain fungi, bacteria, viruses and nematodes are also used to control many insect pests. 

4. Biorational control -- the use of soaps, oil, insect growth regulators and, what is referred to as soft or natural insecticides; those that are friendlier to the environment such as neem oil, pyrethrums, rotenone and Bt.   

Aphids

The new leaves of a citrus tree are all curled.  What is causing this?

During the period of a growth flush, when the new shoots are expanding and the leaves are still in the bud stage, small insects called aphids will feed on these shoots in large numbers.  Aphids have piercing mouthparts, much like a hypodermic needle, extracting vital nutrients from the plant and injecting hormones into the plant.  So when the new leaves finally come out, instead of being large and flat, they are small and curled.  Treating the trees at this point, when the leaves are already showing signs of curling, would be too late. 

Trees need to be inspected when the new shoots are expanding.  The time to treat is when the black, or sometimes, green aphids are seen.  A good treatment for aphids is to spray with a proper horticultural oil or horticultural soap.  The following is a University of Hawai`i recipe which combines both of these active ingredients: first make a concentrate of one tablespoon mild dishwashing liquid and one cup of vegetable oil.   When ready to use, shake well and mix 1 – 2 ½  teaspoons of the this mixture into one cup of water.  Spray plants thoroughly. Repeat sprays may be needed at 5-7 day intervals.  Do not spray in the heat of the day.

Chinese Rose Beetle

It is common to find leaves riddled with holes in an appearance of lace. In addition to feeding by slugs, snail and caterpillars, the Chinese rose beetle (Adoretus sinicus) is the likely culprit.

These beetles especially like beans, cabbage, roses, as well as over 250 commonly grown plants.  The immature stage, called grubs, looks like white worms and lives in loose soil and leaf litter.  The grubs do not feed on leaves but may feed on the roots of some plants.  The adult is nocturnal in habit, remaining under leaf litter during the day and emerging to feed at dusk.  If you are in the garden at this time, and a few hours later, you will be able to see the beetles feeding on the leaves. 

Controlling the beetles is difficult. Sevin is one insecticide that will control Chinese rose beetle but frequent rains lessen its effectiveness.  Neem products can also be used to deter feeding.  A systemic insecticide, with the active ingredient  imidacloprid, works well.  Check labels for registration on food crops.

Beetles are attracted to weak light. Some gardeners have constructed traps utilizing solar powered patio lighting to attract the beetle.  The light is placed over a container of soapy water into which the beetles will fall.  On the other hand, beetles are deterred by bright light. Placing a flood light over the plant will keep beetles away. Lights only need to be on for about 3-4 hours after sunset.

Placing a shade cloth barrier around newly planted trees or areas of the vegetable garden will exclude the beetles from those plants. Newly transplanted trees are more susceptible than older established trees.

Perhaps the surest way of controlling this pest is to go out into the garden after sunset, when the beetles are feeding, and collect them by hand and dispose of them.

Ask the Garden Guy, Science Based Answers to Garden Questions, is an excellent resource book for gardeners. Some popular topics include Slugs and Snails, Organic Pesticides, Reasons, Why Vegetable Seeds Do Not Germinate, What’s So Hot about Manure? Mushrooms in the Lawn.  Purchase by clicking on the image of the book above.