A SPECIES OFPHOMOPSIS AND OTHER FUNGI ASSOCIATED WITH SHOOT DIE-BACK OF PECAN, RELATED TO STRESS AND COLD INJURY
CC REILLY1, BW WOODl and KL STEVENSON2
lUSDA-ARS, Southeastern Fruit and Tree Nut Research Laboratory, 21 Dunbar Road, Byron, GA 31008, USA; 2Department of Plant Pathology, University of Georgia, Athens, GA 30602-7274, USA
Background and Objectives
Spring shoot die-back of pecan, generally attributed to cold injury, becomes obvious within the first month after budbreak. Severity is related to the previous year's crop load and alternate bearing. Thus, trees with heavy crop loads exhibit greater cold damage. Shuck decline, a late season fruit disorder, is most severe on stressed trees. Heavy crop load, premature leaf loss, and excessive rainfall during the early growing season, combined with drought in late August and September increases the severity of shuck decline. Pathogens associated with shuck decline are Giomerelia cingulata and a species of Phomopsis [1,2]. This report presents evidence that this same Phomopsis sp. is also involved in shoot die-back and that the severity of damage is dependant upon cultivar, stress factors and crop level.
Results and discussion
The Phomopsis sp. isolated from pecan shoot die-back produced a concentric colony of light-grey mycelium on PDA at 24°C. As the colony overgrew the Petri plate (6-8 ;days), black conidiomata formed throughout the mycelial mat. Biguttulate aseptate a-conidia (7.9X3.4 ;µm) were produced; p-conidia were not observed in the cultures. The frequency of isolation of organisms from all pecan shoots were similar for both the 1992 and 1994 seasons. Phomopsis sp. was isolated at high frequencies (>90%), for the two seasons. All other fungi, including species of Alternaria, Botryosphaeria, Epicoccum, Fusarium, Glomerelia, Penicillium, Pestalotia and Phoma were consistently low (<10%). Specific cultivars had a range of isolation frequencies for Phomopsis sp. from 76 to 100% in both June 1992 and June 1994. Botryosphaeria dothedia or B. ;obtusa were detected on 10 cultivars in June, 1992 and 13 cultivars in 1994, with isolation frequency ranging from 2 to 22% and 2 to 44% for 1992 and 1994, respectively. Prior to budbreak, shoot die-back was usually less than l0 ;cm, on 1-year-old wood. By June, the die-back extended an average of 33 ;cm into 2-, 3-, and 4-year-old wood.
The Phomopsis sp. was isolated during both the pre budbreak and post budbreak periods from shoots of cultivars Moore, Cheyenne and Moneymaker inoculated the previous season with a Beniate resistant isolate. Beniate-resistant Phomopsis sp. was recovered from 24% of pre budbreak shoots of Moore but none were isolated from the later period. Cheyenne had recovery frequencies of the Beniate-resistant isolate of 64% for pre-budbreak and post-budbreak alike. Seventy three per cent of the Moneymaker shoots had Phomopsis sp. isolated from them with 36% being Beniate resistant.
In stressed trees of cultivar Cherokee, Phomopsis sp. (87%) and Botryosphaeria spp. (10%) were the primary fungi isolated from shoots exhibiting die-back the spring following stress treatments. Eight other fungal species were detected in symptomatic shoots, but isolation frequencies were <=8%. The greatest level of fruit exhibiting shuck decline and percentage of dead shoots was exhibited by trees under the greatest level of stress (maximum crop load and maximum kernel mass demand) whereas decline was least on minimally stressed trees (minimal crop load and minimal kernel mass demand).
These data lead us to conclude that Phomopsis sp. is a stress pathogen capable of causing severe damage to fruit and shoots of stressed pecan trees. Shoot death becomes obvious early the following spring after budbreak, thus accounting for the paradoxical observations of what appears to be winter cold-induced shoot die-back, occurring without much exposure to cold temperature. Thus, we propose that, in many cases, dead shoots exhibiting cold injury are killed by Phomopsis sp. as a result of tree stresses encountered in the previous growing season.
1. Latham AJ, Bowen KL, and Campbell HL, 1995. Plant Disease 79, 182-185.
2. Reilly CC, Reynolds KL, 1994. Proceedings SE Pecan Growers Association, 87, 129-142.