(Top) Shallow roots, caused by a soil pan about 5-6 inches below the soil surface, limited root development and moisture access, causing the roots to grow horizontally and the plants to become stressed. (Middle) Further stressing the plants were stem weevil larvae, who hatch during the summer and tunnel down to the base of the stalk to overwinter, interrupting the plant's vascular flow and weakening the stem. (Bottom) As seen in the photo, stem weevils can literally burrow out a sizeable tunnel in the stalk.

Initial investigation revealed that the harvest was early – perhaps by a month or more. Confection hybrids planted in this area are typically Group 4 and are relatively full season – Group 3 fits best. It appeared that the plants just shut down. Most of the stalks were still standing and the ones that broke off were one to two feet off the ground. The heads were small at harvest.

Digging up the roots started providing clues as to what happened. The roots were shallow with the tap root tending to go off to the side. The evidence suggested a soil pan about 3-5 inches deep. Sunflower’s extensive root system enables it to survive in dry years by efficiently using nutrients (including water). Soil sampling results assume that you can pick up or use the measured nutrients. Soil pans will result in drought symptoms and reduced yields even with plenty of water. The reduced root systems also will make a big stalked flower susceptible to lodging even before wind and snow compound the problem.

The next step in unraveling the mystery of low yields was to look for insects. The primary suspect was the stem weevil. Evidence at this time of year would be larvae in the stalk at ground level. Splitting the stalk revealed 1/4-inch long white larvae at the base of the stalk. The larvae results from the adult laying eggs in late June – any control has to be applied before egg laying. The larvae hatches and begins tunneling down to the base of the stalk to overwinter. The tunnels interrupt the vascular flow and weaken the stalk – both yield-reducing factors. The photos illustrate whey this pest is termed the "corn borer of sunflower." Pure stem weevil damage results in stalks breaking off at ground level. Stalk breakage in this field was one to two feet off the ground and likely had to be caused by something else.

Further investigation found a fungus that was also affecting yields. The stem weevil adult is a vector of Phoma Black Stem and can infect the plant when boring the hole to lay the eggs. Phoma Black Stem fungus overwinters on residue that enters the plant at any opening. According to the High Plains Sunflower Production Handbook, “Larval feeding appears to enhance subsequent proliferation of the fungus within the stalk.” Note the tunneling evident in the photos. This fungus represented the last straw for plant yields.

Phoma Black Stem is the fungus Phoma macdonalddii that overwinters on residue. Symptoms usually appear after flowering at the base of the leaf petioles and spreads along the stems. Given time the black fungus can coalesce into larger blackened areas that weaken the stalk, producing lodging. As it progresses, it girdles the base with a black ring one to two inches above the soil line. This fungus can cause premature ripening and even kill the stalk. The severity depends on when the infection occurred and the degree of the infection.

Stem weevils cause damage two ways — primarily as stalk tunnelers and secondarily as vectors for the fungus, Phoma macdonalddii, which causes Phoma Black Stem.

Recommendation: This field should be out of sunflower for two to three years, if possible. This field and those around it should include the cost of spraying for the stem weevil in the cash flows. Once stem weevils are active in an area, it's best to scout carefully for them.

We often hear how it is not any one thing but often several things working simultaneously to reduce yields. To avoid similar problems in your fields, be sure to reduce plant stress whenever possible so the plant is better able to resist these stressors.

Bill Booker
Extension Educator in Box Butte County

Copyright 2006 by the University of Nebraska Board of Regents. All rights reserved.

Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture.

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