PCC MarchApril Final Draft

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New Findings on Limb Dieback of Figs in California Pruning Practices Can Help Protect from Disease Pathogen By THEMIS J. MICHAILIDES | Plant Pathologist, UC Davis/Kearney Agricultural Research and Extension Center DAVID MORGAN | Staff Research Associate, UC Kearney Agricultural Research and Extension Center and GIORGIO GUSELLA | Graduate Student in Plant Pathology, University of Catania, Italy Back in 2004, and again in recent years, there were concerns by fig growers mainly in Madera and Merced counties about an excessive killing of major branches of their fig trees (Figure 1). Visits to some orchards back then and recently indicated that indeed they had a major problem. Initial close examinations of the dead branches showed symptoms which were similar to another disease: branch wilt of walnut. The bark of dead fig branches had cracks and one could easily remove large pieces of the bark, exposing the woody tissues underneath which were covered by a black powder. Rubbing this black powder with your finger could easily remove masses of it (Figure 2, see page 25). The inner surface of the broken and removed bark pieces were also black due to these powder masses. A lot of trees had many dead major branches while others had one or two dead along with other branches bearing chlorotic and thin canopy, distinct from the green and dense canopy of healthy branches. Figure 1. Left, Fig tree affected by severe limb dieback; top right, still active canker; bottom right, inactive canker (branch is dead) (all photos courtesy G. Gusella.) Pathogen Activity To collect samples, we cut some of the symptomatic branches close to the interface of dead and alive-looking (green) tissues. We noticed that in a cross section, the dead woody tissues were delineated from the healthy tissues by a dark brown line while the living woody tissues were white (Figure 1). Slices of these woody tissues from the branches were taken, isolations were made in the laboratory and a fungus known to be a pathogen of woody tissues was consistently recovered. The name of this pathogen is Neoscytalidium dimidiatum, which is a new taxonomic name of Hendersonula toruloidea fungus, which represents the pathogen first reported to cause the 24 Progressive Crop Consultant March / April 2021
PLANT HEALTH & PEST MANAGEMENT FIGHT WITH BIOLOGICAL BITE Figure 2. Top, Neoscytalidium dimitiatum, the cause of limb dieback producing spores (arthrospores) as the mycelia dried up and separate to small segments under the bark; bottom, easily rubbing off the spores under the bark (photo courtesy Beth Teviotdale.) branch wilt disease of walnut. Checking the literature, the same fungus under a different name (i.e. Nattrassia mangifera) was reported in 1945 on commercial figs in California as well as on Ficus religiosa and Ficus bengalensis, causing dieback and trunk cankers. In addition to walnut branch wilt, which is a common disease of walnuts grown in the San Joaquin Valley, the same fungus was reported in causing branch wilt and dieback of poplar, eucalyptus and mango. In other reports, we found out that this pathogen can cause killing of major branches of walnut, ash trees and grapefruit. More recently, it has been reported causing cankers and hull rot of almond. On fig shoots, the pathogen grows and infects injured bark (i.e. mechanical wounds, wounds by hail or sunburn), invades the woody tissues and kills the branch. When the branch is killed, it dries and usually the bark cracks, exposing large Continued on Page 26 TRANSPLANT POST TRANSPLANT FIRST SIGN OF INSECTS Control downy mildew with Jet-Ag + Stargus Biofungicide, powdery mildew with Jet-Ag + Regalia Biofungicide & aphids, thrips & other insects with Venerate XC Bioinsecticide. Jet-Ag 0.5% v/v + Stargus Biofungicide 2qt./A every 7–14 days Apply 2qt./A every 7-10 days Learn more at marronebio.com/leafygreens rotate with Jet-Ag at 0.5% v/v + Regalia Biofungicide 1 qt./A every 7-14 days rotate with ©2021 Marrone Bio Innovations, Inc March / April 2021 www.progressivecrop.com 25
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