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

IPM IN PECAN PRODUCTION
141
by Fusicladosporium effusum (G. Winter) Partridge and Morgan-Jones. Most years
disease losses are not that high, but growers often spend more money for fungicides
than any other production input. In Georgia alone the cost of scab control is
estimated to be about $15 million annually, and in a wet year like 2003 the
combined cost of lost yield and fungicide sprays was approximately $45 million
(Williams-Woodward, 2003). There are other diseases that can cause local damage
on some cultivars, but they are generally controlled by fungicides applied for scab.
These secondary diseases include Downy Spot (Mycosphaerella caryigena),
Phytopthora Shuck and Kernel Rot (Phytophthora cactorum), Powdery mildew
(Microsphaera penicillata), Zonate leaf spot (Cristulariella pyrimidalis),
Anthracnose (Glomerella cingulata), Bacterial leaf scorch (Xylella fastidiosa),
Crown gall (Agrobacterium tumefaciens), and a number other minor diseases that
rarely cause significant crop loss.
Because losses to disease can be severe under favorable conditions, growers
have a high awareness of the need for control programs. This may include practices
such as pruning lower tree branches or clearing fence rows around orchards,
planting in wide row spacing’s, closely mowing the orchard floor, and designing
orchards so that prevailing winds flow through it (Latham & Goff, 1991). All these
practices are designed to increase air flow, thus reducing leaf wetness periods and
therefore levels of infection. General orchard sanitation practices are also utilized,
and may include shaking previously harvested trees to dislodge shucks that harbor
overwintering pathogen inoculum.
In spite of these practices, growers in areas with frequent rainfall rely heavily
on multiple fungicide applications applied with large air-blast sprayers or
occasionally airplanes. Fungicides used include triphenyltin hydroxide (TPTH),
various sterol demethylation inhibitors (DMI’s), dodine, strobilurins, and
thiophanate methyl. There have been issues recently with fungicide resistance, and
ongoing concerns that it will become an increasing problem (Stevenson, Bertrand, &
Brenneman, 2004). These concerns are based on the known risks associated with the
fungicides used, the inherent ability of the pathogen to develop resistance, the heavy
use of fungicides, and the fact that pecan is a perennial crop and therefore not
subject to crop rotation which can greatly reduce the buildup of resistant isolates.
However, growers generally still get good disease control if they apply sprays
correctly and on a timely basis. Most growers in the southeastern states use a 2–3
weeks spray interval for a total of 7–10 sprays per year, whereas those in more arid
areas of the west may make few if any fungicide applications.
Early sprays (i.e. prepollination) are applied to protect the young leaves which
are very susceptible to scab infections. Overwintered lesions on stems are a major
source of this initial inoculum. Large numbers of conidia are produced from stem
lesions in late March and early April, although small numbers are found as late as
August (Stevenson, 1995). Therefore one function of the early sprays is to prevent
leaf infections that will in turn sporulate and provide secondary inoculum for nut
infections that are even more damaging. Scab susceptibility of leaves decreases
rapidly with age (Gottwald, 1985), although later season growth flushes can become
infected. Late-season fungicide sprays are of questionable benefit when applied after
shell hardening according to work by Gottwald and Bertrand (1989). They