Jason Hartschuh, Extension Field Specialist, Dairy Management and Precision Livestock, Ohio State University Extension
Currently early planted corn is from full pollination to just beginning brown silk, while later planted corn is still in the vegetative stage but will be pollinating over the next couple weeks. The ideal time to scout corn for fungal disease and for fungicide application is from tassel through pollination. Most fields we have scouted this year show low disease pressure, but we have had humid overcast conditions over the last week which may increase disease challenges. We do have a gray leaf spot confirmed in a few locations across the state, and tar spot has been confirmed in southern Michigan.
When fungal disease is present in corn silage at harvest, research shows a fungicide application at VT-R1 to control these fungal diseases reduced fiber concentration and improved nutritional value compared to the untreated control. Corn treated with fungicide had improved fermentation and more consistent dry matter values. When disease was severe, dry matter yield was also improved. When fungal disease infects corn, one of its natural responses to stop the spread of the disease is to increase lignin around the infected area which reduces digestibility.
2021 was the first-year tar spot was found in Ohio, and pressure was low in 2022. Multiple universities across the Midwest are working on tar spot, but we only have limited knowledge on its management and effects on corn silage. Currently, there are no confirmed outbreaks of tar spot in Ohio this year, but a few samples that look like tar spot have been submitted to the lab for diagnosis. It has been confirmed in southern Michigan and southern Ontario, Canada. Previous years have shown us that later planted corn may be at the greatest risk of losses from tar spot. One of the greatest risks to corn silage is from severe infection causing premature plant death. Tar spot reduces silage moisture, digestibility, and energy value which can also lead to poor fermentation with lower silage moisture and plant sugars. Scouting for tar spot is critical and should be done weekly from tassel through R3. Lesions will be small, black, raised spots appearing on both sides of the leaves along with leaf sheaths and husks. Spots may be on green or brown dying tissue. Spots on green tissue may have tan or brown halos. If tar spot is found in fields, a fungicide application can help slow disease spread, but be cautious of the preharvest interval of the fungicide used with some being as long as 30 days and others as short as 7 days. With silage harvesting beginning around R4 growth stage, 50% milk line, the decision to use fungicide should be closely monitored. Another option is to make sure your harvesters are ready if the corn dies prematurely so that silage moisture is at least correct, with tar spot killing plants in less than two weeks under ideal conditions. Tar spot development is driven by cool temperatures (60 to 70oF), humid conditions (>75% relative humidity), and prolonged leaf wetness (>7 hours). These weather conditions often occur over night and in the morning in early August.
Other leaf diseases, such as gray leaf spot, northern corn leaf blight, common rust, and southern rust can also decrease digestibility and fermentation, just not as rapidly as tar spot. Gray leaf spot has gray to tan lesions developing between the veins and are distinctly rectangular with smooth, linear margins along the leaf veins. Lesions are slow to develop, needing 14 to 21 days, and begin in the lower leaves. Northern corn leaf blight lesions typically have a tan color and are elliptical or cigar shaped with smooth rounded ends. Common rust is what we usually have in Ohio, but on occasion, Southern rust may be present. Common rust is rarely of economic concern, but the development of southern rust can have economic yield impacts. The colors are different between the two; common rust is brownish to a cinnamon-brown while Southern rust has a reddish orange appearance. Southern rust mostly develops on the top of the leaf and may be on the stems and husks, while common rust is on both sides of the leaves and generally only on the leaves. The last difference is in shape and distribution; common rust pustules are large and oval to elongated with a scatted appearance over the leaf. Southern rust is small, circular, and evenly distributed over the leaf. Identifying the diseases present in your corn field can help you choose the best fungicide when they are needed.
Each disease has slightly different environmental conditions tin which they thrive, but these conditions can overlap or happen within days of each other. Gray leaf spot is favored by warm temperatures between 70 and 90°F and high relative humidity. Northern corn leaf blight also favors wet conditions but prefers cooler weather of 64 to 80°F. Tar spot is a cool weather disease favoring temperatures from 59 to 70°F during humid conditions of 85% relative humidity or more keeping leaves wet for greater than 7 hours. Another cool weather disease is common rust with optimal disease conditions being temperatures of 61 to 77°F and 6 hours of leaf wetness. Southern rust is more of a late season disease, preferring the warmer temperatures of 77 to 88°F.
Maybe the biggest concern for dairy producers is mycotoxin contamination of corn silage and high moisture corn, with many nutritionists encouraging levels below 1 ppm in corn utilized in the lactating ration. Fungicides may have the ability to reduce mycotoxin levels and improve silage digestibility.
Deoxyinvalenol (DON) is one of the primary mycotoxins in Ohio corn. It is caused by the fungus Fusarium graminearum and causes both Gibberella stalk rot and Gibberella ear rot, making it of concern for both the grain and forage quality. Gibberella ear rot and DON production increase when the 21 days after tasseling are cool and wet. One study under low Gibberella disease pressure, revealed that in all cases but one, an application of fungicide at R1 reduced DON levels by at least 50%. The trial was then expanded for the second year, which was a high disease pressure with DON levels as high as 17.9 ppm in one hybrid and 30.3 ppm in the other hybrid. Again, fungicide had little effect on yield of these two brown midrib (BMR) hybrids or forage quality, but a few products did consistently lower DON levels. A 2021 corn silage trial in Ohio showed a vomitoxin reduction from 3.1 ppm in the control to 0.5 ppm with Miravis Neo application. In 2022, a project at the Northwest Research Station in Custar, OH compared DON levels among 3 varieties and 3 fungicide application methods. There was a difference between varieties in corn silage DON levels, with the varieties containing better disease resistance having lower DON levels. The fungicide application method also affected DON levels, with all treatments being lower than the control. Interestingly, our application over the top at 20 gal/acre had the lowest DON levels followed by drops. The application over the top did a better job of covering the plant from tassel to root with fungicide, which may help with stalk rot management. An earlier project showed better coverage at ear height when using 20 gal/acre compared to 15 gal/acre.
The products that consistently lowered DON levels contained a triazole as one of their active ingredients, with prothioconazole being the most common. Three products that researchers across the country are seeing lower DON levels with when disease is present are Proline, Delaro, and Miravis Neo. The ideal application window from multiple studies has been R1 which is from the point when silks emerge until they become dry about 10 days later. This application is primary for Gibberlla ear rot which infects the ears during pollination. An important part of ear rot control with fungicides is that the fungicide must reach the silk in order to be absorbed at the ear to prevent fungal infection.