Reasons for Poor-Quality Pecans

Prepared by Bob Polomski, Extension Consumer Horticulturist, and Marjan Kluepfel, HGIC Information Specialist Clemson University. (New 03/00.)

HGIC 2213

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Pecans (Carya illinoensis) will grow in almost any soil in South Carolina, except poorly drained soil, hardpan or stiff clays, or thin sands with a high water table. Pecan trees should be spaced at least 40 feet apart to provide sufficient room for future growth. When selecting a cultivar for your landscape, disease resistance is the most important factor.

The pecan is monoecious, that is, the male (catkin) and female flowers are borne separately at different locations on the same tree. The female flowers are borne in clusters near the ends of current seasons shoots in the spring. The catkins are borne on the base of the shoot and along the length of the supporting 1-year-old wood.

Pecans are pollinated by the wind. When the catkins mature, huge quantities of pollen are shed, increasing the chances that the windblown pollen will land on the stigmas of the female flowers. Should the catkins mature before or after the female flower is receptive, pollination does not occur. The fruits develop only after the female flowers are pollinated and the ovules are fertilized by male cells from the pollen.

Usually, within a pecan cultivar, pollen shedding does not closely overlap the period when the stigma is receptive. This condition is called dichogamy, which tends to ensure cross-fertilization. Pecan cultivars differ in the order that the male and female flowers mature. When pollen is shed early, before the female flowers are receptive, the cultivar is called protandrous; when the pollen is shed late, after the female flowers are receptive, the cultivar is called protogynous. Protandrous cultivars are commonly referred to as Type I and protogynous cultivars as Type II.

To ensure cross-pollination, trees of bloom Types I and II should be included in a planting. Trees in the Type I bloom class are Cape Fear, Desirable and Owens. Type II bloom class includes Elliott, Forkert, Gloria Grande and Stuart. Cape Fear is a good pollinator for many other cultivars. It has adequate scab resistance but experiences severe leaf scorch in many locations. Desirable is the cultivar most recommended in the southeastern states, but should not be planted in the Piedmont section of South Carolina. Elliott is very scab-resistant. Forkert has high-quality nuts but is somewhat susceptible to scab in wet years. Gloria Grande is a highly scab-resistant cultivar. Owens possesses good tolerance to scab and other diseases. Stuart is susceptible to downy spot and moderately susceptible to scab.

Care of young, nonbearing trees: To get good growth, prune no more than necessary. The larger the number of leaves left to grow, the more food will be manufactured for more rapid tree growth. Strive for an average terminal growth of about 3 feet annually, by fertilizing in early March with 4 pounds of 10-10-10 plus zinc for each inch of trunk diameter measured a foot above the soil surface. Keep an area at least 6 feet in diameter around the tree cleanly cultivated or mulched to keep down weeds and grass. Many pecan cultivars do not start producing nuts until they are 12 to 15 years of age.

Care of bearing trees: To realize good annual production, trees must be adequately fertilized and insect- and disease-controlled. The recommended fertilizer program is to apply 6 pounds of 10-10-10 fertilizer per inch of trunk diameter. To prevent deficiency, the fertilizer listed above should contain 1 to 2 percent zinc. Lack of enough zinc and nitrogen reduces production more than any other factor!

Zinc Deficiency

Zinc deficiency is called rosette. The most noticeable symptoms of rosette are bronzing and mottling of leaves; early defoliation; dead twigs in tops of trees; abnormally small nuts; small, yellowish, chlorotic leaves; short thin twigs growing on older scaffold branches with rosettes of small, yellowish leaves at the tips. An early sign is a wavy margin on the leaflets.

Alternate Bearing

Alternate pecan production (on and off years) is mainly the result of inadequate fertilization. When trees set a large nut crop and there are not enough nutrients for the nuts to mature and for the tree to store enough plant food production will be low the following year. Early defoliation in the fall usually means no nut crop the next year. Diseases and insects affecting the leaves also contribute to alternate bearing by causing early leaf drop in the fall.

To help prevent alternate bearing, use sound cultural practices. These include disease and insect control, adequate use of fertilizer and zinc, and an extra application of fertilizer in late May or June in years when nutset is heavy.

Probable Causes for Nuts Failing to Fill

Failure of nuts to fill is caused mainly by insect and disease damage to leaves and an inadequate number of leaves. Drought also causes failure to fill, if it occurs late in the growing season.

Premature Loss of Nuts

In many years lack of pollination causes the greatest loss of nuts. Since pecans are wind-pollinated only, excessive rain during bloom prevents pollination, and the unpollinated nuts fall. Weather conditions in some seasons cause the male and female flowers to mature at different periods, and pollination fails to occur. Some cultivars shed their pollen before the female flowers are receptive. To insure pollination, it is important to plant more than one cultivar in an area.

Insects, such as pecan weevil and hickory shuck- worm, and diseases, such as scab mildew and blotch, also can cause premature loss of nuts. More information on pecan diseases is available by requesting fact sheet HGIC 2211, Pecan Diseases.

Drought and too little fertilizer often cause early drop of nuts. Prevent nut loss by harvesting early. Harvesting the nuts as soon as they mature ensures better quality. One of the quickest ways to lose nut quality is to let them lay on wet ground.

Excerpted from EC 678, The South Carolina Master Gardener Training Manual, and from EC 681, Growing Fruits at Home.

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This information is supplied with the understanding that no discrimination is intended and no endorsement of brand names or registered trademarks by the Clemson University Cooperative Extension Service is implied, nor is any discrimination intended by the exclusion of products or manufacturers not named. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.