|Stevenson, K -|
Submitted to: Pecan Grower
Publication Type: Trade Journal
Publication Acceptance Date: January 7, 2010
Publication Date: January 15, 2010
Citation: Wood, B.W., Reilly, C.C., Stevenson, K.L. 2010. Shoot dieback in pecan. Pecan Grower. 21(3):12-13. Interpretive Summary: Limb death maladies are common occurrences in pecan orchards during early spring, potentially resulting in a significant portion of the tree canopy being killed. It was discovered that two major shoot/limb loss maladies are due to a species of Phomopsis fungi which attacks and kills stressed shoots and limbs; hence, minimizing limb/shoot stress reduces tree damage. This information enables pecan farmers to avoid potentially yield limiting death of shoots/limbs in springs by managing in the previous growing season to minimize tree stress.
Technical Abstract: Two shoot dieback maladies (SDM) of pecan [Carya illinoinensis (Wangenh.) C. Koch] are of unknown cause and can adversely affect canopy health. They occur during either early spring (SpSDM) or early summer (SuSDM). Field evaluation found that both maladies predominately occur on shoots retaining peduncles from the previous crop year’s fruit cluster. Isolations of transition zone (from living to dead) tissue of symptomatic shoots, of 14 scion cultivars, found Phomopsis sp. in > 89% of samples, and Botryosphaeria spp. in > 40% of sampled shoots. Isolations occasionally found some combination of eight other fungal genera, with specific genera typically present in < 10% of symptomatic shoots, but were always present in association with either Phomopsis spp. or Botryosphaeria spp. when shoots exhibited either SuSDM or SpSDM. The SpSDM form was associated with > 10 cm of the shoot’s length prior to budbreak in early March before expanding to > 30 cm by late June to produce the SuSDM form; thus, providing evidence for an ongoing and expanding infection common to both SDM forms. The incidence of both “Phomopsis – associated” SDM forms was greatest on trees likely exhibiting substantial crop-associated physiological stress. Evidence indicates that SpSDM and SuSDM forms of dieback are likely two forms of the same malady, and are l caused by Phomopsis spp. and/or Botryosphaeria spp. killing shoots predisposed to damage as a consequence of fruiting associated shoot stress linked to the previous season’s fruit cluster; however, pathogenicity research is needed to determine if they are the primary cause of these shoot dieback maladies. Linkage of Phomopsis spp., and possibly Botryosphaeria spp., to these two SDMs raises the possibility that canopy damage of trees might be reduced by development of management strategies for these fungi.