If you grow corn, you need to learn more about the symptoms to look for to make a correct diagnosis of corn leaf diseases. Goss's wilt is the foliar disease that is on everyone's radar this summer. This yield robber showed up in a number of cornfields in Iowa last year. It is caused by a bacterium, not a fungus, so spraying the corn with a fungicide will not control Goss's wilt.
In the past five years, Goss's wilt, which is a leaf blight, has become increasingly prevalent in Iowa, says Alison Robertson, Iowa State University Extension plant pathologist. This growing season, the disease has already been reported from several fields in the state. Consequently many agronomists and scouts have been checking fields and some agronomists have been making use of the immunostrip test, which is a diagnostic tool available from AgDia.
What symptoms should you be looking for when scouting for Goss's wilt?
Freckles! Look for freckles on the corn leaves, says Robertson. Dark spots that resemble freckles and occur in the outside edge of a developing lesion on the leaf are the most characteristic symptom of Goss's wilt.
If you see freckles, you can be confident you're looking at this disease. However, freckles may be confused with black colonies of saprophytic fungi that colonize dead leaf tissue. "A neat trick we have learned is that if you hold a suspected lesion to the light, freckles appear transparent (see second photo accompanying this article)," says Robertson, "while colonies of saprophytic fungi are dark (see third photo). On systemically infected wilted plants, freckles are also evident on the leaves."
Other characteristics of Goss's leaf blight are very large red to grey lesions that usually start from the leaf tip or leaf margins and extend down the leaf. The lesions are usually a blend of dead tissue, grey-green water soaked tissue and yellowed tissue. You may also notice the lesions are sticky and have shiny patches on the surface, says Robertson. This is the Goss's bacterium that has oozed out of the diseased tissue and dried on the surface of the leaf.
What if you get a positive test result when you use the immunostrip test?
The immunostrip test is a useful test to confirm Goss's wilt, but it must be used carefully, says Robertson. "The test does give false positives," she says. "We know, for example, that purple leaf sheath will give a positive with the strip test from AgDia. Purple blotches on the leaf sheath of corn plants are not disease, but are caused by saprophytic organisms feeding on dust, pollen, etc., that has collected behind the leaf sheath."
Other things you should consider when diagnosing Goss's wilt
"There is other evidence to consider for a positive Goss's diagnosis, in addition to the leaf symptoms and the immunostrip test," says Robertson. Those other considerations are:
* Corn hybrid--Is the hybrid rated susceptible to Goss's wilt?
* Field--Does the field have a history of Goss's wilt? Was corn grown in the field in the previous year and is the crop residue still present on the field surface?
* Weather--Has severe weather occurred in the area within the past few weeks?
"As with all diseases, correct diagnosis is important to enable appropriate management practices to be followed," says Robertson. "For Goss's, planting corn hybrids that are tolerant to this disease is by far the best management tool we have at present. There are numerous foliar-applied products being marketed for Goss's management. Several field trials are being done here in Iowa, and also neighboring states, to evaluate the efficacy of these products."