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Incidence of Scab and
Foliage Condition on Pecan Trees Grown Without Fungicide or Insecticide Sprays
in a Humid Region Additional index words. Carya illinoensis, Cladosporium caryigenum, disease resistance, aphids, defoliation, Monellia caryella, Monelliopsis pecanis, Melanocallis caryaefoliae, pest resistance Summary. Thirty-four pecan [Carya illinoensis (Wangenh.) K. Koch] clones with desirable traits selected from preliminary screenings were evaluated for scab resistance, foliage condition and foliage retention. No fungicide or insecticide sprays were applied in order to increase pest and disease pressure and to better assess suitability of the selections for low input plantings. Most clones were equal to or better than ‘Elliott’, the resistant standard cultivar and were superior to ‘Desirable’, the susceptible standard cultivar, in scab incidence and foliage condition. Pecan trees are frequently grown in small plantings around rural homes in the southeastern U.S. or in urban settings to provide shade, beauty, and nut production. In humid regions, unsprayed trees of most commercial pecan cultivars develop incidence of pests that severely limit both nut production and tree appearance. (Goff et al., 1991). Therefore, most of the popular cultivars like ‘Desirable’ perform poorly, as trees are seldom-sprayed in landscapes or small plantings due to cost or label restrictions. A large pecan tree must be sprayed with an airblast sprayer to obtain good coverage and the cost of these sprayers is not justified by the return from only a small acreage planting. Commercial availability of custom sprayers who will travel to spray small plantings of pecans is very limited. As a consequence, there is a need for development of pecan trees with superior tolerance to pests to fill the niche of pecan trees that perform well in humid environments with little or no spraying. The most serious nut pest is pecan scab Cladosporium caryigenum (Ell. et Lang. Gottwald) which often destroys the crop entirely on susceptible cultivars in humid regions when trees are unsprayed. Serious insect pests include yellow aphids Monellia caryella (Fitch) and Monelliopsis pecanis (Bissell) and the black pecan aphid (Melanocallis caryaefoliae (Davis) (Tedders, 1978). Currently available cultivars with excellent scab resistance all have serious limitations. ‘Elliott’, perhaps the best of available cultivars with outstanding scab resistance, produces small nuts, alternately bears severely, is quite susceptible to yellow aphids, and has very early budbreak making it prone to freeze damage (Sparks, 1992). ‘Gloria Grande’, another cultivar with excellent scab resistance, is extremely susceptible to the black pecan aphid and produces nuts of mediocre to very poor quality in Alabama. Others like ‘Curtis’ and ‘Candy’ produce very small nuts of marginal quality. Cultivars previously thought to have excellent scab resistance, including ‘Sumner’, ‘Melrose’ and ‘Pointe Coupee #2’, have more recently exhibited susceptibility to scab at some locations. The scab fungus exists as numerous strains, some of which may attack a given clone while others will not (Converse, 1960; Turechek and Stevenson, 1998). At a given location, the absence of the strain attacking a test clone may falsely suggest resistance on a cultivar that is readily infected at another location where the virulent strain is present. In an effort to identify pecan cultivars suitable to urban settings and unsprayed plantings in the Southeast, we began selecting, pre-screening, and evaluating pecan clones about 20 years ago. Clones were from a variety of sources: named cultivars, unreleased crosses from the USDA breeding program, native trees selected from humid locations, and "cultivar seedlings" (Thompson and Young, 1985) which are common throughout the southeast (Table 1). These cultivar seedlings are the offspring of improved varieties and often come up in fencerows or fields from nuts carried there by water movement or by birds or squirrels. Another common source of cultivar seedlings is from nursery trees where the scion died after transplanting and the trees grew back from the rootstock thus being offspring of the seed stock source. Commonly, ‘Curtis’ and ‘Elliott’ were used in southeastern nurseries (Grauke and Thompson, 1995) so a great many offspring of these quite resistant parents can be found. The present study reports information on advanced clones which have passed preliminary screening at multiple humid locations with no spraying, and also exhibited good nut quality, excellent pest resistance, and acceptable yields. Materials and Methods Experiment 1: 1996-97 grafts. Grafted trees of ‘Cheyenne’ and ‘Desirable’ growing on rootstocks of unknown origin were planted in 1991-92 at a spacing of 40 x 40 ft (12.2 x 12.2 m) at the E. V. Smith Research Center of Auburn University in central Alabama. In 1997, because emphasis changed to low input in the planting, the trees were re-grafted to test selections exhibiting greater resistance than 'Desirable' or 'Cheyenne' using the inlay bark graft. There were four single-tree replications in a randomized complete block design. Experiment 2: 1999 planting. Bare-root nursery trees were planted at 40 x 40 ft (12.2 x 12.2 m) at the same site as the 1996-97 grafts in the winter of 1998-1999 in skips made from removing trees of clones eliminated from consideration, or from removing trees with grafts that failed. We made the decision to be stringent in our selection procedure and to eliminate cultivars as soon as we found they performed beneath a certain standard based on selection criteria we deemed important. A selection that scabbed at any location as badly as ‘Desirable’, for example, would be eliminated, freeing space in the planting. This selection process created erratic spacing, but up to this point the trees were small and did not fully occupy their allotted space even on the oldest trees, so we did not consider the erratic spacing to have an important effect. In each new successive planting we included ‘Desirable’ as a susceptible check and ‘Elliott’ as a resistant check. In order to reduce any possible effects of erratic spacing, we blocked according to degree of overcrowding so that trees with similar spacing were grouped within a block. There were eight single-tree replications in a randomized complete block design. Experiment 3: 2000 planting. Bare-root nursery trees were planted at 40 x 40 ft (12.2 x 12.2 m) in the winter of 1999-2000, as described for Experiment 2. There were four single-tree replications in a randomized complete block design. Trees in each experiment were fertilized, drip-irrigated, and treated with herbicides according to established recommendations (Goff et al., 1989), but received no pesticide sprays. In early October of each season, trees in each planting were rated for incidence of scab and for foliage condition and retention. Scab on stems was recorded as the number of discernible lesions on the worst-affected foot (30.5 cm) of shoot growth on the tree. Leaf scab was recorded as the percentage of leaf surface visibly affected by scab on the worst affected leaflet on the tree. Nut scab, similarly, was rated as the percentage of nut shuck surface area affected on the worst affected nut on the tree. Foliage condition was a visual rating, on a 1-10 scale where 10 = best, bright green healthy foliage free of damage from pests and 1 = worst, badly damage leaves that have not fallen from tree. Foliage retention was a visual estimate of the percentage of original leaves for the season that still remained on the tree at the time of the rating. Results and Discussion Experiment 1. Table 2 indicates stem scab lesions observed for the 5-year period of 1996-2000 on the trees grafted in 1996. ‘Stuart’ and ‘Desirable’, standard cultivars included as susceptible controls, had higher incidence than any of the test selections. ‘Elliott’, the scab-resistant control, had an unusually high number of scab lesions in 1996, but no incidence in other years. Three selections, ‘Barton’, ‘Curtis’, and USDA 88-7-11 had no stem scab lesions in any of the 5 years. Leaf scab incidence (Table 3) was similar to stem scab incidence, with all test selections being generally low and significantly better than the susceptible controls. Incidence of nut scab was only available on 11 of the 14 selections of Experiment 1 (Table 4), as no nuts were present at season-end on ‘Desirable’, ‘Stuart’, or ‘Carter’. ‘Desirable’ nuts were present early in the 1999 and 2000 growing season, but the nuts were so badly infected by scab that they fell off prior to harvest, which indicates the futility of growing popular cultivars like ‘Desirable’ for nut production in humid regions without fungicide applications. In 1999, only ‘Curtis’ had no nut scab lesions. ‘Gloria Grande’, Hughes, ‘Syrup Mill’, and USDA 82-17-680 had significantly greater nut scab in 1999 than did ‘Elliott’, the scab-resistant control. Environmental pressure similar to that which occurred in 1999 is required to make meaningful distinctions. Rainfall was low in 2000, causing all clones to appear equally resistant. A great number of historical evaluations for scab have been made where pressure was low due to weather or fungicide application. Such evaluations led to false presumptions that selections had acceptable levels of scab resistance when in fact they did not. Examples of cultivars which some researchers and growers considered to have sufficient resistance to grow in the Southeast include ‘Cheyenne’ and ‘Wichita’, and 'Western'. Once these were planted in monoculture, however, incidence even with a good spray program usually increased to the point that economical control was not obtained, and most orchards were subsequently cut down or topworked to more resistant selections. A rigorous screening prior to release could have prevented the widespread planting of and resulting losses from these clones. Similarly, sufficiently rigorous screening at multiple sites could identify clones like ‘Sumner’, ‘Melrose’, and ‘Pointe Coupee #2’, that have resistance at some, but not all locations and are not widely suitable to unsprayed plantings in the southeast, having scabbed badly in certain years at certain sites in our evaluations. Some question whether any pecan clones are resistant to scab, or whether they are all “escapes”, initially lacking exposure to virulent strains. Once the strain is introduced or develops, it might then proliferate when the clone is grown in monoculture in orchards, thereby overcoming what was once perceived as “resistance”. The existence of cultivars like ‘Elliott’, released about 1925 (Sparks, 1992) and commonly grown in the Southeast with only minor scab incidence, is evidence that clones with quite useful and durable resistance may be found. Foliage condition ratings (Table 5) indicate that all of the test selections had better foliage at season-end than did ‘Desirable’ or ‘Stuart’. In the case of ‘Desirable’, which is highly susceptible to scab relative to the clones in these experiments, the low foliage condition ratings could be attributed to leaf scab incidence. In the case of ‘Stuart’, which is not as susceptible to leaf scab as ‘Desirable’ (Table 3), other foliage diseases as well as insect damage likely contributed to the poor foliage conditions at season-end. ‘Stuart’ is highly susceptible to yellow aphids and the pubescent leaf surface is prone to accumulation of sooty mold (Sparks, 1992). The good foliage condition at season-end of most of the experimental clones over several years suggests that they are at least somewhat tolerant to other foliage diseases and to foliage-damaging insects such as aphids and mites. Pecan leaves damaged from pests often senesce and abscise prematurely. ‘Desirable’ had the worst foliage retention in all years, except 1996 and 1997 when it was better than ‘Elliott’ only (Table 6). ‘Elliott’, which has extremely early budbreak (Sparks, 1992), suffered cold damage shortly after grafting and the weakened shoots lost leaves early due to cold damage in 1996 and 1997 and not due to pests. ‘Elliott’ trees recovered from cold damage in 1998, 1999 and 2000, providing marked improvement in foliage retention. ‘Stuart’ trees also had significantly lower foliage retention than experimental clones in 1997 and 1999. Foliage retention ratings may differ among cultivars for two broad reasons (Sparks, 1992). Some cultivars drop foliage prematurely due to nutrient drain from leaves by fruits (Sparks, 1977), however, this occurs primarily on bearing trees with heavy crops. Our report deals with young trees either not fruiting at all or just coming into production and unlikely to have sufficient fruit for this type of stress. The other reason for premature defoliation is pest-related. An assortment of leaf diseases can cause defoliation. Also, pecans damaged by black aphids and pecan leaf scorch mites fall prematurely (Tedders, 1978). Yellow aphid damage, if severe and especially when it is accompanied by sooty mold accumulation, may cause leaves to fall prematurely (Tedders, 1978). Foliage retention is critical in pecan production, as trees which defoliate prematurely have low carbohydrate reserves and nut production is poor in the following season (Sparks and Brack, 1972; Worley, 1971,1979). Experiments 2 and 3. In 1999, little scab was observed on any selection other than USDA 82-17-1614 and ‘Desirable’, which both had higher incidence than any of the other clones in Experiment 2 (Table 7). The 2000 season was dry and incidence of scab was low. In that year, only ‘Desirable’ had significant stem scab lesions in Experiment 2 and 3. Leaf scab incidence (Table 8) was similar to stem scab, with only USDA 82-17-1614 and ‘Desirable’ having appreciable occurrence in either year. Foliage condition (Table 9) was worse in 1999 on ‘Desirable’ than on any other selection, while USDA 82-17-1614 also had a low rating. In 2000, ‘Desirable’, ‘Kanza’, and USDA 88-7-11 had low ratings. Clones in Experiment 3 did not differ in foliage conditions in 2000. Foliage retention (Table 10) did not differ significantly among selections in 1999. In 2000, ‘Prilop’, ‘Mount’, and USDA 89-10-7 retained foliage better than ‘Kanza’ or USDA 77-21-3. We consider low incidence of scab and ability to withstand leaf-damaging pests as prerequisites for cultivar performance in the Southeast. While the selections evaluated here have had at least preliminary indications of acceptable nut quality and yield, most need additional long-term research to document acceptability for release as cultivars. Literature Cited Converse, R. H. 1960. Physiologic specialization in Fusicladium effusum and its evaluation in vitro. Phytopathology 50:527-531. Goff, W. D., J. R. McVay, and W. S. Gazaway (eds.). 1989. Pecan production in the Southeast - a guide for growers. Alabama Coop. Extension Serv. Auburn, Alabama. Goff, William D., Ronnie McDaniel, and Emmett Carden. 1991. Pecan cultivars for landscape and home plantings in the southeastern U. S. Journal of Arboriculture 17(3): 73-77. Goff, William D., M. Nesbitt, R. Mullenax, F. Rasberry, and B. Graves. 1998. Pest resistant cultivars as a way to reduce input costs. Pecan Industry: Current Situation and Future Challenges, Third National Pecan Workshop Proceedings. 79-89. Grauke, L. J. and T. E. Thompson. 1995. Rootstock development. In Wood, B. W., Smith M. W., and Reid, W. (eds.) Sustaining pecan production into the 21st century. 2nd National Pecan Workshop Proceedings. USDA Agric. Res. Serv. ARS-195-3. Sparks, D. 1992. Pecan Cultivars, The Orchard's Foundation. 1st ed. Pecan Production Innovations, Watkinsville, Ga. Sparks, D. 1977. Effects of fruiting on scorch, premature defoliation, and nutrient status of Chickasaw pecan leaves. J. Amer. Soc. Hort. Sci. 102:669-673. Sparks, D. and C. E. Brack. 1972. Return bloom and fruit set of pecan from leaf and fruit removal. HortScience 7:131-132. Tedders, W. L. 1978. Important biological and morphological characteristics of the foliar-feeding aphids of pecans. USDA Tech. Bul.1579. Thompson, T. E. and Fountain Young. 1985. Pecan cultivars - past and present. Texas Pecan Growers Assoc. College Station, Texas. Turechek W. W. and K. L. Stevenson. 1998. Effects of host resistance, temperature, leaf wetness, and leaf age on infection and lesion development of pecan scab. Phytopathology. 88(12):1294-1301. Worley, Ray E. 1971. Effects of defoliation date on yield, quality, nutlet set, and foliage regrowth for pecan. Hort Science 6(5):446-447. Worley, Ray E. 1979. Pecan yield, quality, nutlet set, and spring growth as a response to time of fall defoliation. J. Amer. Soc. Hort. Sci. 104(2): 192-194. 1979. |
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