Bacterial Leaf Scorch (Xylella fastidiosa)
Introduction
Bacterial leaf scorch is caused by Xylella fastidiosa, a xylem-inhabiting bacterium that affects water and nutrient transport into leaves and shoots of mulberry and many other shade trees. The disease occurs during summer season.
Symptoms
The symptoms of disease appear initially as marginal leaf desiccation and inward curling. Marginal leaf browning and undulating leaf necrosis with a distinctive chlorotic halo are the symptom in advanced stage. The necrosis initially starts in the desiccated tissues and spreads to the healthy tissues resulting in a scorched appearance. The affected branches show similar symptoms of die back Vigour of infected plants decline followed by premature defoliation.
Causal organism
Xylella fastidiosa
Systematic position
Description of the pathogen
The bacterium is small (0.25-0.5 x 1.0-5.0 µm), Gram-negative, rod-shaped that lacks flagella for motility and requires oxygen for respiration. The bacterial cells often possess a rippled (undulating) cell wall and terminal fimbriae (surface structures, shorter than flagella, that help to anchor the cells together in the xylem stream). X. fastidiosa, has fastidious nutrient requirements and can be difficult to grow in culture. The bacterium grows slowly on selective medium to form small colonies that appear white to yellow.
Disease cycle
Xylella fastidiosais spread primarily by leafhopper insects (subfamily Cicadellidae) known as sharpshooters, and to a lesser extent, spittlebugs (family Cercopidae). These insects have piercing-sucking mouthparts and subsist on xylem fluid. Both adult and immature (nymph) stages acquire the bacterium when feeding on succulent tissues of infected hosts. As xylem fluid is drawn into the insect, bacterial cells attach to the cibarial pump and the lining of the esophagus (collectively known as the foregut of the insect). There, the bacterium multiplies and forms a biofilm, where it becomes encased and protected in a bacterial “glycocalyx” (made of polysaccharide and protein), extracting nutrients from xylem fluid as it is pumped through the insect. Once an insect acquires the bacterium, transmission to a new host can begin within 1 to 2 hours. In the early stages of feeding, bacterial cells become dislodged and are pumped directly into the xylem where systemic movement within the host occurs. An adult can transmit the bacterium for the remainder of its life, whereas nymphs, which shed the foregut during molting, can do so only until the next molt.
Xylem-feeding insects, particularly leafhoppers, can be polyphagous. Many of the alternative (non economically important) hosts ofX. fastidiosa serve as a food source for potential leafhopper vectors, and many leafhoppers overwinter as adults on these alternative plants. Alternative hosts may be the source of a substantial amount of inoculums. During the infection process, bacteria are transported from host to host by leaf hoppers, spittle bugs, and sharp shooter insects. Once infected, bacteria move down branches; details of this transport through vascular tissue are not fully understood. Once bacteria reach xylem (vascular) vessels, they multiply and are carried throughout the plant fairly quickly.
Predisposing factors
Occurrence of leaf hopper in the field alternative hosts and temperature more than 28°C influence the development of the disease.
Control
Detailed studies are not done on the control of the disease. The diseased plants should be removed and destroyed immediately. Strict quarantine measures should be followed while transportation of plant materials. Disease-free material should be selected for plantation.
Related literature
Gould AB and JH. Lashomb. (2005) APS Feature Story: Bacterial Leaf Scorch of Shade Trees. Online:http://www.apsnet.org/online/feature/bls/
Hopkins DL (1989). Xylella fastidiosa: Xylem limited bacterial pathogen of plants. Annual Review of Phytopathology 27:271-290.
Kostka SJ, Tatta TA, Sherald JL, Hurtt SS (1986) Mulberry leaf scorch, new disease caused by a fastidious, xylem-inhabiting bacterium. Plant Disease 70:690-693.
Sherald JL, Hearon SS, Kostka SJ, Morgan DL (1983) Sycamore leaf scorch: Culture and pathogenicity of fastidious xylem-limited bacteria from scorch-affected trees. Plant Disease, 67:849-852
Sherald JL. (Plant Pathologist, Center on Urban Ecology, National Park Service, Washington, D.C. (1988). New Bacterium Identified as Cause of Scorch Symptoms. American Nurseryman 167(10):15, May 15, 1988.
Wells JM. (USDA, ARS, Plant Pathologist, Rutgers University, New Brunswick, NJ 08903), Boligala C. Raju, Hsueh-Yun Hung, William G. Weisburg, Linda Mandelco-Paul, and Don J. Brenner. 1987. Xylella fastidiosa gen. nov., sp. nov: Gram-Negative, Xylem-Limited, Fastidious Plant Bacteria Related to Xanthomonas spp. International Journal of Systematic Bacteriology, 37 (2) 136-143.