Managing Piedmont Forests To Reduce Losses From the Littleleaf Disease – Southern Pine Beetle Complex

R.P. Belanger – Principal Silviculturist, USDA Forest Service, Southeastern Forest Experiment Station, Athens, GA,
R.L. Hedden – Forest Entomologist, Clemson University, Clemson, SC, and
F.H. Tainter – Forest Pathologist, Clemson University, Clemson, SC.

Integrated Pest Management Handbook, USDA, Forest Service, Agriculture Handbook No. 649, March 1986.

In 1980, the Forest Service and the Cooperative State Research Service of the U.S. Department of Agriculture initiated the Integrated Pest Management Research, Development, and Applications Program for Bark Beetles of Southern Pines. This research/applications effort concentrates on pine bark beetles and associated tree diseases in the South. This is one in a series of Integrated Pest Management handbooks.

Littleleaf Disease – Southern Pine Beetle Interactions

Soil, tree, and stand conditions have to be considered collectively and sequentially in understanding littleleaf-SPB interactions. Littleleaf disease results from the gradual killing of functional feeder roots by the parasitic soil fungus Phytophthora cinnamomi Rands and Pythium spp. There is a close association between the occurrence of littleleaf disease and certain soil characteristics. Littleleaf is most common and severe on sites that are heavily eroded and contain a high percentage of heavy, plastic clays in the surface soil (Fig. 5). Clay soils limit aeration, bind nutrients to soil particles, and contribute to extremes in soil moisture available to plants. These conditions restrict root development and, in turn, contribute to the onset of root diseases. Diseased roots cause physiological stress and a reduction in growth, particularly during periods of drought or excess rainfall.

Figure 5 – Soil conditions can be used to evaluate potential littleleaf sites. (Photo courtesy of Clemson University).

It is important to note that SPB attacks are most closely associated with potential littleleaf sites. The actual presence of diseased trees does, however, further complicate the problem. "Locus" trees – those first attacked and preferred by the SPB – appear to be dominant and codominant trees with root syestems in the beginning stages of decline. Trees in advanced stages of decline are seldom killed by the SPB. It appears that moisture and nutrient supply in these trees may be unsuitable for beetle brood production.

Southern pine beetle infestations occur as two distinct events: initial attack and population growth or spot spread of infestations. The insects first attack stressed trees. After the initial attack, individual spot growth is related to the number of beetles within the area, the presence of nearby newly attacked (attractive) trees, and the density of pines in the stand (Fig. 6).

Figure 6 – More trees are killed in overstocked pine stands in years when beetle populations are high than at any other time.

Figure 7 – Loblolly pine is by no means immune to littleleaf disease. Differences in volume between healthy and diseased trees are greatest in mature stands.

Losses caused by littleleaf-SPB interactions are greatest in shortleaf pine stands. Shortleaf has the widest range of all the southern pines and grows on a wide variety of soils. The growth, yield, and quality of shortleaf pine stands can be excellent on good sites. However, susceptibility to littleleaf increases as site quality declines. Symptoms of growth decline become evident at approximately 25 to 30 years of age. Decrease in vigor is closely associated with increases in stand susceptibility to SPB attack. Loblolly pine, although more resistant than shortleaf pine, is not immune to littleleaf-SPB problems. Symptoms are similar to those on shortleaf pine (Fig. 7), but usually are expressed later in the life of the stand. By this time, growth loss on high-hazard littleleaf sites can be substanital. The littleleaf-SPB complex could become a serious problem throughout the Piedmont where natural stands and plantations of loblolly pine are growing into mature and overmature age classes. Methods are available, however, that enable forest managers to evaluate the extent of the problem in both shortleaf and loblolly pine stands.

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