[Edited_by_A._Ciancio,_C.N.R.,_Bari,_Italy_and_K.(Bookos.org)

IPM IN PECAN PRODUCTION
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1989). The basis for this specificity is not fully known, but it occurs sometime later
than 4 days after inoculation during the stage of subcuticular growth (Bracewell &
Stevenson, 1999). Intercropping a mixture of cultivars has been shown to
successfully reduce epidemics of foliar pathogens in other crops (Mundt, 2002), and
computer simulations based on apple scab show disease reductions of 65–79% after
six generations when planting three cultivars versus a single cultivar (Gessler &
Blaise, 1994). While utilizing that model to guide cultivar placement in new
orchards has been discussed among pecan growers and researchers, the benefits of
planting cultivars in blocks for other management purposes usually outweigh the
potential benefits related to scab control.
The race structure of pecan scab populations and their ability to adapt to new
cultivars has raised questions regarding the objectives of pecan breeding programs
which involve selecting for scab resistance. Breeders have selected for scab resistance
for years, and there have also been attempts to introduce new resistance factors from
other members of the hickory family via interspecific crosses (Graves & Diehl, 1991).
It has generally been recognized that there is a “grace period” after the introduction of
a new cultivar before scab becomes adapted to it, and the length of this grace period
varies considerably among cultivars. One objective of the Georgia breeding program is
to introduce new cultivars with high quality nuts with different resistance genes that
can be transitioned into orchards to maintain diversity (Conner, 2003). The current
Georgia program is also utilizing DNA markers for resistance genes and examining
the physiological basis for scab resistance. Hopefully these efforts will lead to more
durable field resistance in new pecan cultivars.
2.3. Weed Management in Pecan Orchards
Weeds are present throughout the world and many interfere with the optimum
production of food and fiber. Holm, Doll, Holm, and Pancho (1997) state that
approximately 200 plant species world wide are found to cause widespread
problems in our crops. Many of these weeds occur in fruit and nut crops. Pecans are
grown in the southern US and Mexico, an area that accounts for over 98% of the
world’s pecan production (Pena, 2006). Vines, including annual vines such as the
morningglories (Ipomoea species) and perennial vines such as trumpet creeper
(Campsis radicans), and both annual and perennial grasses such as crabgrass
(Digitaria spp.), goosegrass (Eleusine indica), bahiagrass (Paspalum notatum) and
bermudagrass (Cynodon dactylon), are some of the most common and troublesome
weeds in pecans (Southern Weed Science Society, 2006). Other species commonly
found in pecan orchards include nutsedge (Cyperus species), horsenettle (Solanum
species), arrowleaf sida (Sida rhombifolia), spurges (Euphorbia species), horseweed
(Conyza canadensis), and pigweed (Amaranthus species) Colour pictures and
descriptions are found in the Weed Identification Guide published by the Southern
Weed Science Society (2007).
Numerous weed-competition studies have shown the adverse effects of weeds
on agronomic, vegetable, and fruit crops (Zimdahl, 2004). These studies generally
document the effects of individual weed species on the growth and/or yield of a