Raj Singh

Louisiana’s hot and humid climate, long growing season with mild winters and its unique geographical location provide perfect breeding grounds for introduction and establishment of invasive plant diseases. Among some of the most important invasive plant diseases threatening Louisiana specialty crops detected during the past decade are boxwood dieback, citrus canker and palm phytoplasmas.

Boxwood Dieback

Boxwood dieback is a fungal disease caused by Colletotrichum theobromicola. After its first detection and report in 2015, a National Pest Alert to help support initiatives to track, report and manage invasive pests was developed in collaboration with the North Central Integrated Pest Management Center in 2017. Because this is a new emerging disease, a complete diagnostic guide including all aspects of boxwood dieback was published to assist other professionals in accurate identification of the disease. A molecular identification procedure using a real-time polymerase chain reaction platform — a technique used to amplify small segments of DNA — has been developed to help boxwood growers accurately detect the pathogen at early stages of disease development. Research conducted by the LSU AgCenter Plant Diagnostic Center on host range and in-vitro fungicide screening revealed that all boxwood cultivars are susceptible and only four out of nine fungicides are highly effective in controlling the pathogen. The disease has been confirmed in 20 Louisiana parishes, including Rapides, where many nurseries specialize in boxwood. A hands-on training geared toward identification and management of boxwood dieback was attended by 33 professionals in Rapides Parish, and an identification card differentiating boxwood dieback from other boxwood ailments was produced for Louisiana residents.

Citrus Canker

Citrus canker is a highly infectious bacterial disease caused by Xanthomonas axonopodis pv. citri. Currently, the parishes of Jefferson, Lafourche, Orleans, Plaquemines, St. Bernard, St. Charles, St. James and St. John the Baptist are quarantined. The disease has also been identified from parts of East Baton Rouge and Livingston parishes. The AgCenter Plant Diagnostic Center continues to provide primary molecular diagnostic support for citrus canker disease to Louisiana Department of Agricultural and Forestry and U.S. Department of Agriculture. Citrus tissue collected as part of the Cooperative Agricultural Pest Surveys are tested at the Plant Diagnostic Center using a real time polymerase chain reaction assay to detect the presence of the bacterium responsible for causing citrus canker. Of all the citrus grown in the state, satsumas are the most popular and commonly grown cultivar in Louisiana. A two-year field study was conducted to screen the commercially available satsuma cultivars to citrus canker under natural disease pressure at three different locations, and the results showed that Brown’s Select and Miho were significantly less susceptible compared to other cultivars. Planting of these less susceptible satsumas with timely application of copper-based fungicides before disease development may help manage citrus canker effectively. An extension fact sheet explaining the results from this study was produced for Louisiana citrus growers.

Palm Phytoplasmas

Two new fatal diseases of palms have been detected in Louisiana — lethal bronzing and lethal yellowing. Both are terminal diseases, and symptomatic palms that tested positive or palms with dead spear leaf must be removed immediately. Lethal bronzing is caused by a phytoplasma, Candidatus Phytoplasma palmae 16SrIV-D, an unculturable bacterium with no cell wall that is closely related to — but genetically distinct from — phytoplasma strain 16SrIV-A, which causes lethal yellowing.

Lethal bronzing was first detected in 2013 from Phoenix canariensis in Orleans Parish. In 2017, Chinese windmill palm (Trachycarpus fortunei) was reported as a new host of lethal bronzing disease from East Baton Rouge Parish. Lethal yellowing was detected from East Baton Rouge and Jefferson parishes on T. fortunei and P. sylvestris in 2018 and 2019, respectively. A disease survey was conducted from 2015 to 2017 in southern Louisiana to determine the geographic distribution and host range of palm phytoplasmas. During this survey, P. canariensis was the most affected palm species in Louisiana with lethal bronzing. Currently, lethal bronzing is present in East Baton Rouge, Iberia, Jefferson, St. Bernard and West Baton Rouge parishes, and lethal yellowing is present in East Baton Rouge and Jefferson parishes.

In 2020, the AgCenter collaborated with the Louisiana Department of Agricultural and Forestry to revise lethal bronzing and lethal yellowing quarantine regulation guidelines for Louisiana. In 2021, the AgCenter contributed to the revision of U.S. Department of Agriculture Animal and Plant Health Inspection Service Plant Protection and Quarantine Division Pest datasheets on these diseases. These datasheets provide pest-specific information to support planning and completing early detection surveys.

Raj Singh is an associate professor and director of the Plant Diagnostic Center in the Department of Plant Pathology and Crop Physiology.

(This article appears in the winter 2022 issue of Louisiana Agriculture.)

Figure 1: A boxwood showing random dieback of twigs with tan colored foliage caused by Colletotrichum theobromicola.

Figure 2: A sweet orange fruit exhibiting raised corky lesions caused by Xanthomonas axonopodis pv. citri.

Figure 3: Two dead Canary Island date palms affected with lethal bronzing phytoplasma.

Figure 4: A Chinese windmill palms exhibiting dead spear leaf caused by lethal yellowing phytoplasma.

Figure 5: Current distribution of boxwood dieback, citrus canker, lethal bronzing and lethal yellowing diseases in Louisiana.