Winter 1994 (v6n1)
A.R Nyczepir, RF. Bertrand, B.M. Cunfer, R.W. Miller and R.C.
Gueldner
6th Stone Fruit Decline Workshop, USDA-ARS, Byron, GA. (In Press)
Researchers in Georgia have tested numerous cover crops for their
effect on ring nematode populations (Criconemella xenoplax)
and a related disease of peaches known as peach tree short life
(PTSL). Of the species tested, 'Stacy' wheat showed the greatest
promise in suppressing the nematode. This paper summarizes results
of on-going research into pre and postplant uses of this wheat
variety to reduce the disease.
Peach Tree Short Life and Bacterial Canker
In the southeastern U.S., the second largest peach producing region,
PTSL is taking a huge toll on trees just coming into production.
In South Carolina alone, the disease killed 1.5 million trees
between 1980 and 1990, resulting in a loss of about $6.3 million
per year In California, the disease is known as bacterial canker,
or bacterial blast. While not as serious as in the southeast,
bacterial canker does reduce production. It also affects other
stone fruits, almonds, and pears.
Symptoms. The disease is expressed in several different
ways. Darkened cankers may appear on the Internal bark, which
often exude balls of amber-colored gum. In severe cases, the entire
tree may collapse, during which time the bark emits a vinegarlike
odor In the blast phase, buds in the lower portion of the tree
may fail to grow in the spring, or young shoots or blossoms may
die suddenly.
Control. Control of this disease has been based on choice
of rootstock and fumigation. Plum on peach (especially 'Lovell')
rootstock is less susceptible than on plum rootstock. Georgia
researchers have developed a peach rootstock that appears to be
highly resistant to PTSL. Ring nematode predisposes trees to PTSL,
so orchard soil is usually fumigated before planting. However,
with the impending loss of methyl bromide and the rising interest
in sustainable farming practices, nonchemical means of controlling
the nematodes (in this case cover crops) are being explored.
Cover Crop Research
This paper presents preliminary results from several three-year
research projects.
Small Grain Replant Study. In this experiment, from 19861989,
plots were planted to: 1) 'Stacy' wheat (no fumigation), 2) 'Lovell'
or 'Nemaguard' rootstocks (fumigated vs nonfumigated), or 3) fallow-disked
(no fumigation). In 1990, all plants were removed and 'Loring'/'Nemaguard'
peaches were planted to all plots. In 1991 and 1992, nematode
densities and incidence of PTSL were substantially greater in
nonfumigated peach plots than the other treatments (table 1).
The data also showed that, while ring nematodes were found in
all plots in the second year, growing 'Stacy' wheat for three
years was comparable to preplant fumigation in reducing incidence
of PTSL.
Rotation Study. The purpose of this study was to the effects
of three-, two- and one-year plantings of 'Stacy' (prior to peach
planting) on ring nematode densities. Treatments included: 1)
Wheat+fallow, 2) a wheat+sorghum double crop (using a sorghum
cultivator that is a poor host to ring nematodes), and 3) peach
alone, nonfumigated. The 3-year, 2-year, and 1-year sequences
were initiated in the late fall of 1990, 1991, and 1992, respectively.
In order to provide a food source for nematodes before the 2-
and 1-year rotations were initiated, peach seedlings were planted
in the first year of the study; they were removed before initiating
each of these treatments. The entire test site will be planted
to peaches in the winter of 1994.
Initial results show that both two years and one year of each
wheat planting greatly reduced nematode densities compared to
peach alone (table 2). (Reviewer's note: In the rotation plots,
nematode densities remained at a level that could be economically
damaging in a fumigated orchard. The reason is that, in fumigated
soil, ring nematode densities may increase drastically with little
competition or natural enemies and ample peach roots for food.
The effects of these nematode densities on trees in an unfumigated,
cover-cropped orchard may be less because natural enemy populations
would likely be higher.)
Wheat Interplanting Study. The researchers tested the ability
of 'Stacy' wheat to control nematodes as a postplant management
strategy. The treatments used were wheat, herbicide alone, and
herbicide plus postplant nematicide (phenamiphos). The wheat was
planted in five-foot strips on either side of the tree row. After
two years, none of the treatments were adequately reducing nematode
populations.
Root Exudate Study. This study evaluated attraction and/or
repulsiveness of 'Stacy' wheat and 'Nemaguard' peach root extracts
to ring nematodes. Root extracts were prepared, freeze-dried,
and redissolved in water. The solution was poured onto a paper
disc, which was placed on either end of a rectangular strip of
water agar in a petri dish. Seven nematodes were then placed on
a drop of water in the center of the agar strip. Nematodes were
subjected to the following choices: peach vs. water, peach vs.
wheat, wheat vs. water, water vs. water. Wheat extract was not
repulsive to nematodes, since more nematodes migrated toward wheat
when presented with wheat or water. Peach, however, was more attractive
than either wheat or water.
After two years of the study, it appears that 'Stacy' wheat is
demonstrating more potential as a preplant rather than a postplant
management strategy for the ring nematode and PTSL.
| Table 1. Postplant population density of ring nematodes and incidence of PTSL in small grain plots replanted entirely to peaches ('Nemaguard' rootstock). | ||||||
| Previous | Number of nematodes per 100 cm 3 soil | Incidence of PTSL(%) | ||||
| plant species | Cultivar | Fumigation | Mar. 1991 | April 1992 | May 1991 | May 1992 |
| Peach | Nemaguard | no | 134 a1 | 310 a | 46 a2 | 50 a |
| yes | 0 b | 18 b c | 8 b | 8 b | ||
| Lovell | no | 108 a | 138 a | 58 a | 67 a | |
| yes | 0 b | 4 c | 8 b | 13 b | ||
| Wheat | Stacy | no | 0 b | 49 b | 4 b | 13 b |
| Fallow | no | 0 b | 39 b c | 8 b | 13 b | |
| 1 Mean separation by Duncan's Multiple range test (P< 0.05).
2 Mean separation by Fisher's Exact test (P< 0.01) | ||||||
| Table 2. Results from the first two years of wheat rotations on ring nematode population density. | |||
| Rotation sequence | |||
| First two years of | |||
| three-year rotation | |||
| W + S - W + S | 23 | b | |
| W + F - W + F | 30 | b | |
| First year of two-year rotation | |||
| W + S | 20 | b | |
| W + F | 38 | b | |
| One-year rotation | |||
| no data as of publication | |||
| Peach (continuous) | 399a | ||
References
Nyczepir, A.P. and P.F. Bertrand. 1990. Host suitability of selected
small grains and field crops to Criconomella xenoplax.
Plant Dis. 74:698-701.
Stanley, D. 1993. Preserving Georgia's peach pride. Agric. Research
41(5):10-15.
For more information write to: Southeastern Fruit & Tree Nut
Research Laboratory, P.O. Box 87, Byron, GA 31008.
(Cl-FMCC.085)
Contributed by Chuck Ingels
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