Bacterial leaf streak, caused by Xanthomonas vasicola pv. vasculorum (syn X. campestris pv zeae), was confirmed for the first time in the United States in 2016, and the disease has been confirmed in Colorado. The disease has only been reported on corn previously in South Africa, although the pathogen has been reported as causing a gumming disease on sugar cane in other countries. The species has undergone several name changes and is recognized by other synonyms including X. axonopodis pv. vasculorum and X. campestris pv. zeae.
Fig. 1. Orange-brown lesions with wavy margins of bacterial leaf streak caused by X. vasicola pv. vasculorum (Courtesy K. Broders)
Symptoms of this disease on corn plants are narrow, wavy-edged lesions that range in length from less than an inch to several inches long (Fig 1 -2). They may be tan, brown or orange in color and occur between the veins of the leaf on the leaves of corn. Sometimes the lesions occur close to the midrib; in other cases, they occur across the leaf blade. When back lit, light passes through the lesion in a translucent manner (Fig. 3). Disease symptoms have been observed as early as growth stage V7 with lesions appearing on lower leaves first. Lesions can expand over time to cover larger areas and under favorable conditions, they spread to the upper leaves. In extreme cases, lesions may extend the entire length of the leaf and coalesce to form large, necrotic areas.
Fig. 2. Narrow elongated lesions of bacterial leaf streak on corn (Courtesy K. Broders)
Disease Cycle and Epidemiology
There is currently limited information about this disease and what impacts it may have on corn production. There is also little known about the epidemiology of the pathogen. For example, it is not known where it survives the offseason, how and when initial infection occurs, or how it is secondarily spread. It is presumed that X. vasicola pv. vasculorum over seasons in infected crop debris and is splash dispersed to leaves of healthy plants during early stages of growth. Research has not been conducted on whether or not it is seedborne. As with other bacterial diseases such as Goss’s bacterial blight, weeds adjacent to corn fields may serve as alternate hosts. Research is currently under way to address each of these aspects of the X. vasicola pv. vasculorum life cycle.
It does not appear that wounding is required for entry into the plant by the bacterium. It most likely enters through stomata and then moves through intercellular spaces. Irrigation during hot weather appears to increase the disease incidence. The bacterium is also likely spread via rain splash and wind dispersal, but it is unclear how far the bacterium may be able to travel.
Fig. 3. Translucent symptoms of bacterial leaf streak lesions when back lit (Courtesy A. Robertson)
Several other corn diseases can cause similar symptoms which can complicate diagnosis. Lesions caused by X. vasicola pv. vasculorum look very similar to gray leaf spot (GLS), except that they have wavy margins compared to gray leaf spot lesions that have very straight sides and are more rectangular in shape (Fig 4a & b). When GLS lesions are backlit (Fig 4a), light does not easily pass through the lesion and they are more opaque compared to the translucent lesions caused by X vasicola pv. vasculorum. The other way this bacterial disease can be distinguished from other fungal diseases is by the presences of bacterial streaming. Bacterial streaming occurs when a small piece of infected tissue is placed in a droplet of water on a microscope slide. When viewed under low power on a compound microscope, the bacterial cells of X. vasicola pv. vasculorum can be seen streaming out from the edge of the lesion in large quantities. For more information on distinguishing BLS symptoms from those of other corn pathogens, check-out this flyer from the University of Nebraska.
Bacterial streaming from a corn leaf infected by X. vasicola pv. vasculorum, observed under a microscope (Courtesy R. French)
Fig. 4a. Back lit Grey leaf spot lesions with a more rectangular and opaque lesion (Courtesy A. Robertson)
Fig. 4b. Rectangular symptoms of Grey Leaf Spot on corn (Courtesy A. Robertson)
Currently there is limited research on management strategies for this disease. Field observations suggest that there are differences in susceptibility among corn hybrids. Once hybrids can be screened for resistance, use of resistant or more tolerant hybrids will be the way to manage the disease. Like other bacterial diseases such as Goss’s blight, no effective chemical controls currently exist. Until more research has been conducted to determine the most effective management strategies for this disease, corn producers are advised to use standard management practices for bacterial disease. This will include the following
- Sanitation practices to remove any infected debris from equipment between fields in order to slow the spread of the pathogen
- Use of crop rotation or tillage to reduce the amount of infected corn debris and reduce the survival of the bacteria
While both of these strategies may reduce the amount of the pathogen present, neither will eradicate the bacteria and eliminate the risk of disease.
Dr. Kirk Broders – Colorado State University
Dr. Jan Leach – Colorado State University
Jillian Lang – Colorado State University
Dr. Tamra Jackson – University of Nebraska-Lincoln
Dr. Kevin Korus – University of Nebraska-Lincoln
Dr. Alison Robertson – Iowa State University
Dr. Doug Jardine – Kansans State University
Dr. Ron French – Texas A&M University