Wednesday | November 22, 2017
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Best methods for controlling plant diseases

There are many approaches when dealing with diseases in the garden. I will go through a number of them and then tell you the very first step you need to take.

A. Cultural control: Grow healthy plants. Stressed plants are more susceptible to diseases and insects too. Keep plants well-nourished and watered. Sanitation is a part of cultural control — keeping the growing area clean of diseased plant residues. Crop rotation is also used for controlling soil borne fungi attacking plant roots.

B. Physical and mechanical control: This involves the removal of diseased leaves or an infected limb and disposing of them properly. At times, the entire diseased plant will need to be removed from among the many healthy ones. Foliar diseases, such as the ones attacking tomatoes, can be minimized by placing a roof over the crop. This will restrict the rain which would normally spread the disease by splashing the fungal spores from leaf to leaf and plant to plant.

C. Biological control: Utilize certain good fungi, bacteria and other organisms to feed on and destroy plant pathogenic fungi, bacteria, etc. This method is not as widespread in managing diseases as it is in combating insect pests.

D. Chemical control: This is probably the best all-around material for the control of foliar fungal and bacterial disease is the application of a copper based pesticide. This material is used as a protectant rather than eradicating the disease.

All of the above methods should be in the gardener’s arsenal for controlling disease in the garden. The best approach, however, is to begin by using resistant varieties.

Cultural, mechanical, physical and chemical methods require work and in many situations money.

The one simple step that may negate all the leaf pulling, cleaning, building and spraying is to begin with seeds that are resistant to the disease in question.

Check with seed companies to see if resistant seed is available. This applies very nicely to the vegetable garden, and when planting fruit trees or even some ornamental, there are varieties that are immune or less prone to certain diseases.

To that end I would like to direct you to the Cornell University Vegetable MD online. Go to and click on the “Resistant Varieties” on the sidebar. Here you will find a list of vegetable crops, from beans and cabbage to squash and zucchini.

Choose the crop you are interested in and up comes an excellent chart listing the numerous crop varieties and their resistance to various diseases and physiological disorders. Most importantly, the seed companies which sell the specific variety are noted.

Search the phone number or website for the specific company and order your seeds.

Planting resistant varieties will go a long way in making life simpler in the garden.

Please note, however, that the word “resistant” means just that; it does not mean immune. It can also mean that in tests, the resistant variety simply did better than nonresistant varieties.

Spending a little time before you plant will save you lots of time and money after you plant.


Can I plant a macadamia nut and get a good producing tree?

Macadamia trees grown from seed will grow into a nice looking tree, but most of the time the nuts will have lower oil content, inferior flavor and production will be poor.

When buying fruit trees, including a macadamia tree, buy a grafted, budded or air layered tree, not a seedling. There are some exceptions including papayas, coffee, jaboticaba and mangosteen, which are usually grown from seed.

Most citrus seedlings, perhaps 80 percent, will run true, but it is still a good idea to buy a budded tree so you get a rootstock that is resistant to root decaying fungi and viruses.

Macadamia varieties recommended by University of Hawaii College of Tropical Agriculture and Human Resources (CTAHR) include Purvis, Ka‘u, Kakea, Keaau, Mauka, Pahala and Makai.

Hilo resident Nick Sakovich is a professor emeritus of the University of California. He has worked in the field of agriculture for 30 years. Email your questions to Sakovich at


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