Bacterial Late-Season Decline Disease of Corn

Bacterial late-season decline is a newly identified bacterial disease of corn first documented by scientists at the Texas A&M AgriLife Experiment Station at Bushland, Texas, in 2021. Researchers observed the sudden appearance of drought stress-like symptoms on previously healthy corn. Testing of symptomatic plants found that the presence of a bacterial pathogen was consistently associated with symptomatic tissues.

Partial gene sequence analysis of the bacterium found it to be very closely related to Pantoea ananatis, a known pathogen of several plant species – including corn – and close relative of P. stewartii, the causal pathogen of Stewart’s wilt. Subsequent research confirmed the bacterium recovered from symptomatic plants in the field to be the causal pathogen of the disease. However, observed symptoms were distinct from those previously reported for either P. ananatis or P. stewartii in corn (Obasa et al., 2023), which is why it is has been designated a new disease.

Late-season decline has shown the potential to significantly reduce corn yield, with up to 90% loss reported in the worst cases. It also appears to be spreading. Foliar symptoms similar to those observed at Bushland were observed in a field in Potter County in 2021. In 2022, the disease was found in all 22 counties of the Texas Panhandle (Obasa, 2024).

Early-season symptoms during vegetative growth stages tend to be localized to the leaves. Leaves of infected plants have light green, elongated, slightly translucent, non-chlorotic streaks with non-wavy margins (in contrast to Stewart’s wilt, which produces lesions with wavy margins). Foliar symptoms at this stage can vary and may be overlooked or misdiagnosed. Vegetative stage infections can also cause stunting of the plant and delayed tasseling (Obasa, 2024).

As plants reach reproductive growth stages, symptoms can become much more severe. Lesions on the leaves can coalesce as they become larger and eventually turn necrotic, resulting in leaf blight symptoms that can resemble severe drought stress (Figure 1 and Figure 2). Other symptoms during reproductive growth include senescence of the tassel, as well as small ears with few or no kernels on them. Stalk rot has also been associated with late season decline in some cases (Obasa, 2024), which can lead to a higher incidence of stalk lodging.

The exact identity of the bacterial pathogen that causes late season decline has not yet been determined. Partial gene sequence analysis of the bacterium found it to be very closely related, but phylogenetically distinct from P. ananatis (Obasa et al., 2023). At this point, it is unclear whether the pathogen is a strain of P. ananatis or a newly discovered species within the genus Pantoea. The Corteva Agriscience Diagnostic Lab does not currently have molecular tools to distinguish the late-season decline pathogen from P. ananatis, assuming that a such a distinction exists.

Pantoea ananatis is a very unusual plant pathogen because it can occupy a wide range of ecological niches other than plants and does not always function as a pathogen. In some cases, it can even be beneficial to plants by promoting plant growth or protecting the plant from disease by fending off other pathogenic bacteria and fungi. P. ananatis is considered an emerging plant pathogen because of its increased incidence of plant disease, host species, and geographical range over the past few decades (Coutinho and Venter, 2009).

P. ananatis is a ubiquitous bacterium, found in nearly every environment on Earth. It is a common epiphyte of plants, living on the external surface of plant tissues without causing any detrimental effects to the plant. In several cases, P. ananatis has been shown to have antibacterial and antifungal properties, protecting host plants against disease caused by other pathogens.

P. ananatis can also be an endophyte, living inside plants, where it can be beneficial, neutral, or a latent pathogen, depending on the scenario. In pepper plants, it has been shown to have plant growth promoting properties and induce systemic resistance to bacterial spot disease caused by Xanthomonas axonopodis. In rice, it can function as an endophyte, but it can also be a pathogen, causing palea browning and stalk rot.

P. ananatis has been found in a diverse range of environments functioning as a saprophyte, living off dead or decaying organic matter. Its habitats include rivers, soil, plant rhizospheres, and in the guts of several species of insect.

The first documented case plant disease caused by P. ananatis was fruitlet rot in pineapple in the Philippines in 1928 (Serrano, 1928). Since this initial discovery, it has been documented as a pathogen in several other plant species as well and can cause a wide diversity of disease symptoms. In corn, P. ananatis has been previously reported to cause necrotic or white leaf spots or streaks (Krawczyk et al., 2021) and stalk rot (Goszczynska et al., 2007). Leaf blight of corn caused by P. ananatis has been documented in South Africa, Brazil, Mexico, and Poland (Krawczyk et al., 2021).

P. ananatis is known to be transmitted by insects. It has been found in the gut microbiome of several insect species. Field research conducted in Poland found that western corn rootworm (Diabrotica virgifera virgifera) can be a vector of P. ananatis in corn (Krawczyk et al., 2021). In onions, it can be transmitted by tobacco thrips (Frankliniella fusca) (Gitaitis et al., 2003).

Much remains unknown about late-season decline, including sources of inoculum, mechanisms of transmission, and epidemiology of the pathogen. The bacterium that causes late-season decline appears to be an opportunistic pathogen. In samples analyzed by the Corteva Agriscience Diagnostic Lab, P. ananatis has been detected in diseased plants as well as healthy plants exhibiting no symptoms of bacterial disease. Opportunistic infections are infections caused by bacteria, fungi, viruses, or commensal organisms that normally live in or on a host organism without causing disease but become pathogenic when the organism’s defense system is impaired. What triggers the bacterium to become pathogenic in corn is unknown.

Observations and yield data from late season decline cases thus far indicate that the yield impact of the disease can vary widely depending on multiple factors (Obasa, 2024). Timing of infection and overall plant health can influence disease impact on yield. Generally, the earlier infection occurs, the greater the likely impact on the plant. The presence of other stressors may also increase the susceptibility of plants to infection. Environmental conditions appear to play a significant role in determining impact on the crop. Late-season decline is favored by high temperatures and moisture, as is generally the case for bacterial diseases. Overhead irrigation can create a favorable microclimate for the proliferation of bacterial disease. Fields under irrigation with a dense canopy, where moisture can persist on the plants for extended periods of time, are at higher risk. Initial field observations also suggest that corn hybrids can differ in their susceptibility to late-season decline.

Resistant corn hybrids are the main tool for managing bacterial diseases. Ratings for genetic resistance to late-season decline have not been developed yet; however, continued occurrences of this disease should provide the opportunity to collect hybrid performance data under natural disease pressure. Management of irrigation timing and frequency may also be a useful tactic to mitigate the spread of late-season decline by reducing the duration of wetness in the canopy. Foliar fungicides are not effective since this is not a fungal pathogen.

• Coutinho, T.A., and S.N. Venter. 2009. Pantoea ananatis: an unconventional plant pathogen. Molecular Plant Pathology. 10:325-335.
• Gitaitis, R.D., R.R. Walcott, M.L. Wells, J.C. Diaz Perez, and F.H. Sanders. 2003. Transmission of Pantoea ananatis, causal agent of center rot of onion, by tobacco thrips, Frankliniella fusca. Plant Dis. 87:675-678.
• Goszczynska, T., W.J. Botha, S.N. Venter, and T. A. Coutinho. 2007. Isolation and identification of the causal agent of brown stalk rot, a new disease of maize in South Africa. Plant Dis. 91:711-718.
• Krawczyk, K., J. Forys, M. Nakonieczny, M. Tarnawska, and P.K. Beres. 2021. Transmission of Pantoea ananatis, the causal agent of leaf spot disease of maize (Zea mays), by western corn rootworm (Diabrotica virgifera virgifera LeConte) Crop Protection. 141:105431.
• Obasa, K., M. Kolomiets, B. Reed, D. Coker, J. Bell, and K. Heflin. 2023. Late-season decline – A new bacterial disease of corn identified in the Texas Panhandle. Plant Health Progress. 24:198-206.
• Obasa, K. 2024. Late-season decline disease of corn. PLPM-PU-107. Texas A&M AgriLife Extension.
• Serrano, F.B. 1928. Bacterial fruitlet brown-rot of pineapple in the Philippines. Philippine J. Sci. 36:271-324.