July Disease Outlook
With slim margins in markets right now, it is more important than ever to protect your investment. You have already been through fertilizing, planting, side dressing, topdressing and pre and post herbicide application. The last thing you want is for it all to be for naught, lost to some deadly disease outbreak like we have seen in years past. Although our conditions may have not exactly been ideal for disease formation thus far, we have had some heavy rains, some heavy dew mornings, and a lot of humidity. As a result, there are already cases of Northern Corn Leaf Blight popping up. It is important to continue to scout your fields to know what to look for and where to look.
NCLB (Northern Corn Leaf Blight) can rob a corn plant of 30% of its yield potential if the plant is infected before tasseling. NCLB shows up as tan/brown oval-shaped lesions on the leaves. Remaining on corn residue from the previous year, NCLB infects this year’s crop by being splashed onto lower parts of the growing plant and then continues to infect upward. For this reason, the first signs of disease are commonly observed on the lower leaves. Heavy dews, frequent rains, high humidity, and moderate temperatures help spread the disease from plant-to-plant and from field-to-field. If the days turn dry and hot, NCLB will slow its progression, however, it will resume spreading if conditions once again become favorable.
On the other side of the weather spectrum, common rust usually favors cooler, wetter conditions. Common rust fungi produce fungal spores that look like raised bumps on the leaf surface. These raised pustules can be rubbed off the leaf and leave a brown or orange stain on fingers or clothing. Common rust pustules appear on the upper and lower leaf surfaces and are usually brown to brownish red, elongated in shape, and scattered. Rust spores infect Iowa fields by blowing up from the warmer southern climates. Approximately six hours of dew is enough moisture for infection and disease development. Rust diseases are parasites and divert nutrients for fungal growth that would normally be used by the corn crop. This process can lead to yield loss during grain fill when the plant resources are needed to contribute to the developing ear. If the rust pustules get rubbed off the leaf, it can sometimes be confused with another common disease, gray leaf spot.
Gray leaf spot shows up on plants first as small lesions surrounded by yellow halos. As the lesions mature, they elongate into narrow rectangular, brown to gray spots. Gray leaf spot spreads much the same way as northern corn leaf blight; it overwinters on crop residue and in the spring the spores splash onto the new crop. For this reason, it normally starts on the lower leaves of the plant. Infection of gray leaf spot occurs during prolonged, warm and humid periods. Symptoms are commonly observed following long periods of heavy dew and overcast days. The lesions created by gray leaf spot reduce the amount of photosynthetic areas on leaves available which contribute carbohydrates to the developing grain. This reduced photosynthetic area can also contribute to stalk rot and lodging.
As always, knowing your fields is the first step to controlling and spreading disease. If you know your hybrids are susceptible to any of these diseases or if you are on a corn-on-corn crop rotation, make sure you are scouting your fields. Look at the lower leaves of plants to see if they are infected. If temperatures stay wet or humid, continue to scout your fields to look for disease symptoms. If you do notice something in your fields, fungicide can be applied for in-season disease management. We have seen time and time again that fungicide applications pay for themselves in years where outbreak is widespread. This can be the difference between profitable fields and losses. To get added to our fungicide application list or for more information on disease management, contact your Stutsman Agronomist.