Figure 1. Southern rust of corn is characterized by small orange pustules that aggregate on the upper surface of the leaf. It also can be found on the husks of the ears and stalks, and sometimes pustules are surrounded by yellow halos.
- Southern rust is a fungal disease that must blow into corn production areas in the mid-North parts of the United States each year
- When Southern rust arrives prior to R3, and conditions are conducive, significant yield losses can occur
- Southern rust has recently been detected at low levels in several states in the Midwest region, including Missouri, Illinois, and Kentucky
- Proper disease identification is key to if/when to apply a fungicide to maximize ROI
Southern rust activity has started to increase in corn fields located of the southern Midwest and southeast portions of the United States, with recent confirmations of the disease in Western Kentucky, Southwest Missouri, and Southeast Illinois in the last week. Although the current amount of disease is very low, it is important to keep an eye on our fields, especially fields at high risk for this disease, which include late plantings and plantings near areas of detection. We also want to make sure we aren’t sounding the alarm on southern rust when other less important issues may be present and misidentified as southern rust.
Biology and disease identification:
Southern rust is caused by the obligate fungal pathogen Puccinia polysora and needs a living host to grow and reproduce. Each year, the disease needs to reestablish itself in corn production systems in the United States through moving north on air currents from warmer regions. Conditions that favor disease development include hot temperatures (morning low of 75°F and daytime high of 93°F) and at least 4 hr of consecutive leaf wetness (Table 1). This is a very short period of leaf wetness, and often will be met by accumulated dew on fields during nighttime periods. Consequently, unlike common rust, which is favored by cool, wet conditions, Southern rust can continue to build and develop during hotter summertime months. After infection, new pustules containing a large quantity of infective spores will form in approximately 7 days. Spores will be dispersed and potentially infect new corn tissues and plants. The infection-spore production-dissemination-infection cycle will repeat itself if environmental conditions are favorable and susceptible corn is infected and still green. When corn dries down and is harvested, the disease cycle stops until it blows in the next year.
Identification:
Puccinia polysora produces fuzzy, raised structures called pustules on leaves, husks, and stalks of corn. Each pustule contains thousands of small orange spores. When you rub these pustules between your fingers, the spores may leave a dusty orange coat on your fingers, hence the reason it is called a “rust.” Pustules of Southern rust are orange to light tan, and often small and circular, and may have a “halo” around them (Figure 1, 2). Pustules are mostly found aggregated on the upper leaf surface, which can help distinguish it from the less damaging common rust. Common rust is favored by cool, wet weather and frequently is only observed early and very late in field corn when conditions favor development (Table 1). Consequently, it is not active when we are at our most sensitive time, from, VT-R4. In addition, most hybrids have good resistance to common rust, but only some have moderate resistance to southern rust. Therefore, the combination of the ability of Southern rust to persist in hot conditions and the lack of highly resistant corn hybrids, makes this an important disease to identify when it starts in fields.
What other things that might look like southern rust?
Physoderma (leaf stage) (Figure 3). This is a disease that infects during very wet conditions when the plants are still in the whorl. Some hybrids present a barber shop pole striped leaf infection near VT from these earlier infections. No spores/pustules are present, purple dots often accumulate on the midrib, and often infection is fairly spotty and scattered in a field (and plant). It will not spread during the season and does not need a fungicide application.
There are numerous other issues that can look like southern rust and are not of a concern. Contact your FS Crop Specialist if you think you have southern rust, so you can be confidant in your agronomic decisions.
Management
In general, a fungicide application consisting of at least two modes of action is recommended when southern rust is detected in a field and the crop has not reached R3. Remember, foliar fungicides containing multiple active ingredients provide roughly 3 weeks of protection, and optimal applications for foliar fungal diseases is between VT and R2. However, if southern rust arrives in fields prior to VT, an earlier application may be warranted. Make sure to base all decisions on in-field scouting, crop growth stage, and economic considerations.
Table 1. Comparisons between Southern rust, a disease of considerable concern, and common rust, a disease that often is not of consequence in corn production.
Figure 2. A single pustule of the southern rust pathogen. Note the halo and round shape of the pustule.
Figure 3. Common rust is characterized by red oblong pustule on the upper and lower sides of the leaf. The disease is favored by cool temperatures and often shuts down in the middle of the growing season when temperatures increase. Most hybrids have excellent resistance to common rust. Image CPN.
Figure 4. The foliar stage of Physoderma brown spot can easily be misidentified as southern rust. Foliage often has symptoms in bands on the leaf and purple/black spots that aggregate on the midribs. Foliar and stalk symptoms are often banded due to the light requirements that the pathogen needs to infect leaves. This infection took place in the whorl and does not need to be addressed.
