Association of Neonectria macrodidyma with dry root rot of citrus in California

Authors: ADESEMOYE, ANTONY - University Of Nebraska; HAJERI, SUBHAS - Central California Tristeza Eradication Agency; Yokomi, Raymond - Ray; ESKALEN, AKIF - University Of California

Submitted to: Journal of Plant Pathology & Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/20/2017
Publication Date: 1/22/2017
Citation: Adesemoye, A., Hajeri, S., Yokomi, R.K., Eskalen, A. 2017. Association of Neonectria macrodidyma with dry root rot of citrus in California. Journal of Plant Pathology & Microbiology. 8:391. doi:10.4172/2157-7471.1000391.

Interpretive Summary: Increased land costs, new citrus varieties, and use of semi-dwarfing rootstocks have contributed to the current trend of high-density planting of citrus. To manage tree growth, microjet irrigation/fertigation, regular pruning, and/or hedging are used. Although high density plantings contribute to early positive return in fruit production and economics, long-term effects on citrus trees may be compromised due to excessive irrigation/inadequate drainage and/or poor soil/pH conditions. Recently, reports of dry rot of citrus associated with young bearing citrus trees (3- to 10-year old) grafted on trifoliate or hybrid rootstocks (e.g., C35 and Carrizo citrange) have increased in California. The causal agent of citrus dry rot is Fusarium solani, a ubiquitous and generally weak soil pathogen. Under stressed conditions, however, citrus trees become vulnerable to F. solani infection through wounds in the root system. Since 2010, systematic surveys in central California citrus groves were conducted and the fungus, Neonectria macrodidyma (formerly Genus Cylindrocarpon), was consistently recovered from citrus with dry rot symptoms. Herein, it is reported that a Neonectria fungus was associated with citrus dry root rot in central California. This pathogen causes black foot disease of grapevine in the U.S. and many other countries. Considering the wide host range and damage that this fungus may cause to citrus, further studies are warranted to better understand its epidemiology and biology in citrus and to determine if it causes a synergistic interaction with other citrus fungi such as Phytophthora nicotianae (syn. P. n. var. parasitica), P. citrophthora, and F. solani. If it does contribute to susceptibility of citrus to dry rot, additional control measures will be needed to improve wood- and soil-borne disease management practices.

Technical Abstract: The fungal genus Cylindrocarpon (teleomorph: Neonectria Wolenw.) include ubiquitous soilborne pathogens that cause black foot disease on a wide range of hosts, including grapevine, strawberry, apple, and conifers. Hosts typically become infected through natural wounds on roots and other below ground parts. During surveys in 2010 and 2011 for dry root rot diseases of citrus in California, samples with root rot symptoms were collected in Tulare County citrus orchards. Small pieces of tissue from root samples were plated onto potato dextrose agar amended with 0.01% tetracycline and incubated at 25 degrees C. Characterization of the fungal isolates was done by morphology and by sequencing of the Internal Transcribed Spacer and Beta Tubulin sequences amplified by PCR. Neonectria macrodidyma (Cylindrocarpon macrodidymum) was first recovered in 2011 and has been recovered consistently since then from citrus samples with dry root rot symptoms. As this pathogen appeared to be widely distributed in association with citrus dry root rot, it may interact synergistically with Fusarium solani, Phytophthora nicotianae and P. citrophthora, major soilborne pathogens that cause root and/or crown rot diseases of citrus in California. Considering the damage that N. macrodidymathis may cause to the citrus industry, detailed studies are warranted to better understand its distribution, epidemiology, and pathogenicity to improve disease management practices.