Individual bearing pecan [Carya illinoinensis (Wangenh.) C. Koch] trees periodically exhibit episodes of substantial shoot and small limb death referred to here as “shoot dieback maladies” (SDMs). These SDMs have received little study and are not usually considered of economic importance; however, in severe cases, they likely contribute to economic loss of tree canopy structure and photoassimilation capacity. Such loss can potentially influence nutmeat yield, quality, and alternate bearing (Wood, 1995; Worley, 1979a, 1979b). One SDM form occurs during early spring about, and just after, budbreak; as a result, it is here termed “spring shoot dieback malady” (SpSDM). The second SDM form occurs during early summer and is here termed the “summer shoot dieback malady” (SuSDM).
The cause(s) of these maladies are unknown; however, extension specialists and farmers typically attribute the SpSDM to “winter cold injury” and the SuSDM to “shading” (authors’ experience). Both explanations are questionable when SpSDM occurs in the absence of abnormal winter or spring cold, and the SuSDM often occurs on limbs exposed to full sunlight, thus raising the possibility that pathogens are involved. At present, pathogens are not typically recognized as contributing to cold injury-like symptoms in pecan, although Cole (1968) and Matz (1918) noted a possible linkage between Botryosphaeria spp. and putative cold injury dieback. Also, Glomerella cingulata, a pathogen associated with fungal leaf scorch and anthracnose (i.e., shuck disease; Latham et al., 1995; Rand, 1914; Reilly and Reynolds, 1994; Sparks et al., 1995) might also be associated with these SDMs. A dearth of information regarding a potential causal role for microorganisms merits study of pathogens associated with symptomatic tissues of the shoot dieback maladies. The present study reports the types of fungi present in symptomatic tissues of SpSDM- and SuSDM-associated shoots and reports that SpSDM is linked to degree of physiological stress experienced by shoot structures. We report evidence that these maladies are tightly associated with and are likely caused by Phomopsis sp. and that severity is influenced by both host genotype and previous tree stress.
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