Garden Guy: Heavy rains leech nutrients from soil


With the recent storms we have experienced this month, it’s very likely that many of the plant nutrients have leached from the soil.

Nitrogen fertilizers are water soluble and are especially vulnerable to being leeched below the root zone.

Potassium, although not as soluble as nitrogen, will also leech, especially in courser textured soils.

Phosphorous, the third major plant nutrient, does not move readily with the water, but this element is often lacking in availability in many soils in Hawaii. Depending on your particular soil analysis, now would be a good time to apply either a complete fertilizer or a high nitrogen fertilizer.

Remember all fertilizer packages will have three numbers printed on them, as an example, 10-15-5, denoting the percentages of the three primary plant nutrients, nitrogen, phosphorus and potassium. In the above example, the fertilizer would contain 10 percent nitrogen, 15 percent phosphorus and 5 percent potassium. This is important to know because different plants and different situations require different types of fertilizer.

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Garden Guy, I was able to grow great green beans but the leaves of the plants look worse each day. They have yellow spots all over. Sending you a photo. What can I do? I just planted more young plants. Will this happen to them also? W.U.

From the photo, you have a disease called halo blight, caused by the bacteria, Pseudomonas syringae pv. phaseolicola. It is spread from leaf to leaf and plant to plant in windblown rain and irrigation water. The bacteria enter the plant through wounds and natural openings when the foliage is wet and during periods of high humidity. It affects the foliage as well as the seed pods.

The first symptoms are small lesions resembling tiny pinholes on the underside of leaves. They rapidly dry and turn reddish brown and become visible on both sides of leaf. These spots usually remain small, but a characteristic yellow zone, resembling a halo, often develops around the lesions. They can be of irregular size and shape. Symptoms on the bean pods begin as tiny spots gradually enlarging to form dark sunken lesions. Infected pods are discolored and shriveled.

Control: There is some resistance to this blight. For instance, navy and small white bean cultivars along with some red kidney bean cultivars are resistant to halo blight. Check for resistant varieties in the type of bean you desire. Since the disease is spread by rain, it would help to provide some type of cover that would lessen water droplets falling on the plants. Although these products will not eradicate the disease, copper based bactericides can reduce infection and protect healthy tissue. Do not plant beans in the same location for three years.

The leaves of my tomato seedlings are riddled with tunnels. Yet I’ve noticed after planting in my garden, the tomatoes do fairly well. What is causing this?

The tunneling is caused by the immature, or larval stage of a group of insects called leaf miners, which for the most part are flies (order Diptera). The tunnels are made as the larvae feed between the upper and lower surface of the leaf, meandering their way about. Sometimes, black thread-like strips of frass, insect droppings, can be seen in the tunnels.

Gardeners can tolerate the damage of most leaf miners that occurs because of the several species of parasites (the good guys) which attack the larvae while they are feeding within the leaf tissue. Many times, damage is confined to the seedlings. Once they are set out, the plants tend to outgrow the pest, while the parasites take over and keep damage to a minimum. Chemical sprays are generally not recommended because they would kill the parasites along with the pest. In addition, the larvae are well protected within the leaf tissue. Some gardeners pick and discard the infested leaves. This may not be a good idea because inside those leaves many parasites are waiting to emerge to attack more leaf miner larvae.

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 askthegarden guy@earthlink.net. You also can visit his website at www.gardenguyhawaii.com.

 

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