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.