A major drought occurred in the southeastern United States from 1999 to September 2002. During spring 2002, branch death occurred in the tops of some pecan trees. Severity of a dieback varied from south to north Georgia. The purpose of this study was to determine whether a relationship existed between dieback severity and rainfall during developmental phases of the pecan tree.

Dieback was visually rated in mid-May 2002 as none, mild, or severe. Trees in the none category had no visibly dead branches but shoot growth in the tree tops was suppressed resulting in a thin appearance of the upper canopy. On trees in the mild category, dieback was mainly limited to one-year-old branches. Dieback did not occur on all trees. In the severe category, dieback symptoms were relatively consistent among trees and extended into 6-year or older branches (Fig. 1).

Mature trees were rated at Dawson, Montezuma, Fort Valley, Forsyth, Athens, and Commerce, Georgia. Ratings were restricted to non-irrigated trees growing on abandoned home sites, roadsides, and in abandoned orchards. The ratings were a consensus of observations of multiple trees (40 or more) in each location. Dieback in spring 2002 was assumed to be a culmination of three consecutive drought years. For this reason, degree of dieback was compared with the sum of rainfall departures from normal for 1999 to March 2002. Rainfall departures were from recording stations (National Climatic Data Center, 1999-02) located in close proximity to the towns.

Rainfall departures were calculated for varying time spans that were (1) annual, (2) November to March, (3) April to September, (4) April to May, (5) June, (6) July and August, (7), September, and (8) October (Table 1). Except for annual, selection of these time spans was based on the developmental phases of the pecan tree. During dormancy (November-March), fall and winter rains normally replenish the soil profile. April to September is the principal growing for pecan. In April and May, the spring flush of shoot and leaf growth occurs (Davis and Sparks, 1974), June coincides with early fruit elongation (Sparks, 1986), July and August coincide with fruit expansion (Sparks, 2000), and September encompasses most of the kernel filling period (Davis and Sparks, 1974). Fruit maturation occurs in October and, usually, October is the last significant month of the growing season.

Dieback was not correlated with annual rainfall. Annual rainfall departures in Dawson, Montezuma, and Commerce were similar at -21 to -23 inches (Table 1). In spite of the similar rainfall deficit, dieback was radically different. Dieback was mild in both Dawson and Montezuma, but severe in Commerce.

Fall and winter rains occurring from November to March did not account for dieback severity (Table 1). Pecan trees in the Fort Valley area did not have dieback. However, areas with lower and with similar winter rainfall deficits to Fort Valley had dieback.

Dieback was not correlated with rainfall during the major portion of the growing season of pecan from April to September (Table 1). Dieback was severe in Commerce, yet rainfall was sightly above normal. Likewise, trees in Forsyth and Athens had severe dieback, but had rainfall deficits of -25 and -11 inches, respectively.

Dieback was not correlated with rainfall during the spring flush of growth occurring in April and May (Table 1). Dieback was mild in Dawson and Montezuma, but severe in Athens despite all three areas having similar rainfall deficits.

Rainfall during early fruit elongation (June) was the only period during which rainfall was above normal at all locations (Table 1). Dieback was poorly correlated with rainfall; Athens and Commerce, with severe tree dieback, had substantially higher above-normal rainfall than other areas. Similarly, dieback was not correlated with rainfall departure during fruit expansion (July and August). Athens and Commerce had normal or above normal rainfall and severe dieback; whereas, rainfall in other areas was deficit. Only during the period of kernel filling (September) were rainfall departures correlated with tree dieback. Rainfall departure was highest (i.e., wettest) in the area with no dieback (Fort Valley), intermediate in areas with mild dieback (Dawson and Montezuma), and lowest (i.e., driest) in areas with severe dieback (Forsyth, Athens, and Commerce). Rainfall was 12 inches above normal in the area with no dieback, 5 to 8 inches in areas with mild dieback, and near normal in areas with severe dieback.

Data in Table 1 suggest that tree dieback following the severe 1999 -2002 drought was induced by differences in rainfall during the kernel filling period. Furthermore, these data suggest that the effects of a severe drought can be greatly negated by above normal rainfall during the kernel filling period. The apparent relationship between dieback and rainfall during kernel filling is as might have been anticipated. Kernel development is the most stressful stage of fruit development (Davis and Sparks, 1974); over fruiting in very prolific cultivars can result in dieback or tree death (Hunter and Hammar, 1948), and kernel development stress is apparently accentuated by soil moisture deficit (Sparks, 1996). Additionally, September is normally the second driest month of the year in Georgia and in about 50% of the years there is insufficient rainfall for optimum kernel development (Sparks, 1995).

In the southeastern United States, near maximum kernel filling is associated with 1.25 inches of rain per week during the first half of September (Sparks, 1992). During the 1999-2002 drought, average weekly rainfall for the first half of September for Fort Valley, Dawson, Montezuma, Forsyth, Athens, and Commerce was 2.0, 1.1, 0.7, 1.0, 0.6, and 0.6 inches, respectively. Thus, according to this standard only Fort Valley had sufficient rain for maximum kernel filling. Under arid conditions, the calculated water requirement for the pecan tree, as a whole, during September is 2.0 inches per week (Miyamoto et al., 1995). Similar calculations for Georgia show that for severe drought areas the tree’s average water requirement in September from 1999 to 2001 was also 2.0 inches per week. During the 1999-2002 drought, average weekly rainfall in September for Fort Valley, Dawson, Montezuma, Forsyth, Athens, and Commerce was 1.5, 1.0, 1.2, 0.8, 0.8, and 0.9 inches, respectively. With respect to the tree’s water requirement in September, all locations were deficient in rainfall even though rainfall was near or above normal. Although no dieback occurred in Fort Valley, the upper canopy was thin (Fig. 1) suggesting 1.5 inches of rain per week in September was inadequate for optimum tree growth and productivity.

The normally dry Georgia September where pecan is not native contrasts with the wetter Texas September where pecan is native (Sparks, 2000). Higher September rainfall in its native areas ensures species survival via well-filled seeds and maintenance of tree vigor.

Normal annual rainfall for Fort Valley, Dawson, Montezuma, Forsyth, Athens, and Commerce is 44.6, 49.9, 45.7, 49.1, 49.7, and 53.7 inches, respectively; normal rainfall in September is respectively 2.8, 3.5, 2.6, 3.0, 3.3, and 3.6 inches. Normal annual rainfall and September rainfall in areas with severe dieback (Forsyth, Athens, and Commerce) are higher than in the area with no dieback, Fort Valley. Thus, the association of dieback with rainfall in September (Table 1) was not due to an accentuation of differential normal rainfall among study areas.

The data in Table 1 have implications for pecan irrigation in the humid southeastern United States. Soil moisture is especially critical during the period of kernel filling, as has been repeatedly emphasized (Sparks, 1992; 1995; 1996; 2000, 2003). Normal rainfall in September in Georgia is inadequate for optimum tree performance. Although the tree’s water requirement varies with evapotranspiration, maximum water requirement appears to be near 2.0 inches per week during September. Thus, if irrigation allocations become restricted in the southeastern United States, maximizing irrigation during kernel development will produce greater economic returns and better tree vigor than maximization at any other time of the year. In years with rainfall deficits in June and July, restricting irrigation to September will result in small nuts (Sparks, 2000) but they are more likely to be well filled (Alben, 1957; Sparks, 2003). In years with normal and above normal rain in June and July, nut volume will be average and above average respectively. Under these conditions, maximizing soil moisture in September is essential because a large volume nut is more difficult to fill with kernel than a small volume nut (Sparks, 2003).