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Extension Agent: Sorghum downy mildew found in local corn field

April 26, 2010 at midnight
Updated April 26, 2010 at 11:27 p.m.

This corn plant in Victoria County shows sorghum downy mildew's bleaching effect on leaf color. This plant will probably not produce an ear of corn.

By Joe Janak

A plant disease, sorghum downy mildew, recently developed in a corn field in Victoria County.

This disease, caused by a fungus in the soil, occurred in the same field when sorghum was grown on the site in 2008.

Sorghum downy mildew was a serious disease in the Upper Coast counties during the 1960s and early 1970s, but since then, its incidence has diminished due to fungicide treated seed.

Unfortunately, the seed treatment used in the past is not used anymore, so sorghum downy mildew disease may increase again.

In the affected corn field, the incidence of downy mildew is less than 5 percent. Infected plants are generally stunted and are lighter green than healthy plants - symptoms that also resemble a nutrient deficiency. Leaves also have pale-green areas close to the stem, with a fuzzy, white growth on the underside.

Infected corn plants may or may not develop ears. Infected sorghum plants do not develop seed.

The good news is that if you haven't seen this disease in your corn or sorghum by now, you will not get it this season.

Our current recommendations are that if the disease is present in a field, rotate out of sorghum for at least two years and plant a resistant hybrid in the future.

Crops such as soybeans, cotton and rice are not susceptible and can be used in rotations.

For more information, contact the extension office.

Parasitic weed found locally

I was recently alerted to a very unique weed that shows up periodically. Not only is this weed on the state/national invasive list, but it is also a parasitic weed of other plants.

This weed has no leaves and looks like orange-colored hair growing on other plants. It is called dodder and was more prevalent in the '50s and '60s, when legumes were grown more.

Dodder only lives for a few days in the soil when the seed germinates, and then it disconnects from the soil and gets all its nutrients being a parasite of the host plant.

Dodder cannot be controlled with conventional herbicides. In fact, the best way to control dodder is to kill the host plant. In many cases, that may be the plant you are trying to grow.

Typically, we see a 10- to 15-foot circle of dodder in an area.

In this local case, the dodder is growing on wildflowers and weeds in a 45-acre pasture between Victoria and Nursery. It has already established itself rather well on about 15 acres.

The local dodder is nearly at the seed production stage now, so stopping it quickly could prevent further spreading and infestations.

Killing the host plants (wildflowers/weeds) with hormone herbicides (such as 2, 4-D and others) or glyphosate (Roundup and others) was an option about a month or two ago. At this stage, it will take 10-14 days to kill the plant and then another week for the dodder to die, and by then, the dodder has already produced viable seed.

The best advice now for quick control to prevent seed formation is to totally destroy the host plant (and the dodder) by multiple diskings or tillage passes.

If you see this invasive parasitic weed on your place, destroy it and the host plant as soon as possible.

Joe Janak is a Victoria County extension agent.

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