GEORGIA / FLORIDA
UNIVERSITY OF GEORGIA
Department of Agronomy, Griffin, GA 30212, USA.
J.W. Johnson, R.D. Barnett, B.M. Cunfer, G.D. Buntin, and D.C. Bridges.
The 1998 Georgia winter wheat crop was grown
on about 300,000 harvested acres, which was the lowest in 20 years.
The crop production resulted in a state average yield of 44 bu/acre.
Acreage was down due to wet conditions at planting. Overall, the
season was characterized by a mild, wet winter followed by a dry,
hot spring. Some cultivars had vernalization problems due to the
warm autumn and winter and delays in planting. Cool and dry conditions
prevailed through the grain-filling stage. Septoria glume blotch
and leaf rust also were major factors in some low yields. Hessian
fly infestations reached damaging levels in some parts of the
state. Biotype L expanded its range in the state. Cereal leaf
beetle also reached damaging levels this year, resulting in the
spraying of several thousands acres.
Breeding. The SRWW line GA 89482 will be increased for release.
GA 89482 was selected from the cross Pioneer 2555/PF 84301//FL302,
which carries the T1BL·1RS translocation. This cultivar
possesses excellent resistance to powdery mildew, moderate resistance
to leaf rust, and good resistance to Hessian fly. GA 89482 has
excellent test weight and straw strength and is medium in maturity.
Research. Adult-plant resistance to powdery mildew was
investigated for three resistant wheat cultivars, Knox 62, Massey,
and Houser, and the susceptible Chancellor. Massey was the most
resistant followed by Knox 62 and Houser. The resistance genes
in Knox 62 are mainly additive, and seven chromosomes are involved
in the adult- plant resistance. Chromosomes 5A, 7A, 4B, and 5D
have genes for positive effects on resistance, whereas chromosomes
3A, 1B, and 1D have factors increasing susceptibility. The segregation
of the disomic F2 populations and noncritical crosses fit a four-gene
model, suggesting that chromosomes 5A, 7A, 4B, and 5D each contain
one gene or one cluster of tightly linked genes.
The influence of temperature on the resistance to powdery mildew
was evaluated in six wheat cultivars/lines, Axminster/8* Chancellor
(Cc) (Pm1), Chul/8*Cc (Pm3b), Yuma/8*Cc (Pm4a),
VPM1 (Pm4b), Kavkaz (Pm8), and Chancellor, at two
temperatures of 25·C and 15·C with two isolates
of powdery mildew. Resistance, based on the components of colony
appearance period, infection type, colony number of unit leaf
area, infected leaf area, and AUDPC, was much less effective at
15·C than at 25·C for VPM1 and 'Yuma/8*Cc', but
no resistance shift was observed for Kavkaz and 'Axminster/8*Cc'.
This indicated a difference of temperature-sensitivity among Pm
genes or among the interactions of individual host genes with
corresponding powdery mildew pathogen genes.
In recent years, Georgia wheat farmers have increasingly experienced
difficulties controlling Italian (annual) ryegrass in wheat with
Hoelon (diclofop). An option is to use Sencor (metribuzin) as
a postemergence treatment for control of annual ryegrass, wild
radish, and other broadleaf species. Some wheat cultivars are
sensitive to Sencor (metribuzin), and applicationscan cause severe
injury and/or stand loss. The Sencor label contains a list of
known tolerant and sensitive cultivars of wheat and barley, but
does not include all varieties that are currently grown in Georgia.
Field trials conducted at Griffin, GA, included approximately
40 wheat cultivars that were submitted to the Statewide Variety
Testing Program for testing and consideration for recommendation.
Metribuzin was applied postemergence at a rate of 0.375 lb ai/acre
in water at 20 GPA.
Severe wheat injury and stand reduction were observed with many
cultivars, including previously known sensitive cultivars and
some previously untested cultivars. Of the 37 cultivars tested,
17 were categorized as tolerant, which indicates that total crop
injury did not exceed 10 %, and detectable stand reduction occurred
compared to the untreated paired plot. Four cultivars were categorized
as moderately tolerant, which indicates that crop injury ranged
from 11 to 20 % and that stand loss was insignificant. Sixteen
cultivars were categorized as sensitive, which indicates that
crop injury exceeded 20 % and/or significant stand loss was observed.
Seventeen of the 37 cultivars tested are currently listed as either
tolerant, moderately tolerant, or sensitive on the Sencor label.
Plant Pathology. Bunt in annual ryegrass (Lolium multiflorum)
seed is caused by Tilletia walkeri, a newly described
Tilletia species, and was found in wheat fields at 13 locations
in south central Tennessee, northern Alabama, and Georgia. Teliospores
of the fungus were found at several additional sites. These collections
areidentical to isolates of the smut fungus found on ryegrass
seed in Oregon. Ryegrass bunt is a cryptic disease found at very
low levels throughout the southeastern U.S. In all but one collection,
only a few fully or partially bunted seeds were found. At one
site in Georgia, about 50 % of the ryegrass seed was partially
bunted, and a low percentage of fully bunted seeds was found.
Even at this site, bunted ryegrass was not recognized by visual
inspection of the dry seed. Ryegrass bunt was found in the same
areas where smut spores were detected on wheat seed in USDA surveys
in 1996-98. Ryegrass bunt is the source of spores found on wheat
seed in USDA surveys in 1996 that were initially misidentified
as Karnal bunt. No wheat with Karnal bunt or spores of the fungus
has been found in the southeastern U.S.
Inoculum of Stagonospora nodorum was present in wheat fieldthroughout
the year except in midsummer s as indicated by infection of wheat
seedlings. Seedlings 17 m from the edge of a field were infected
infrequently. The results suggest that inoculum travels only a
short distance from wheat plants or stubble. Therefore, conidia
are probably the major sources of inoculum, and it is unlikely
that ascospores play a significant role in Georgia.
Entomology. The management and use of selected insecticide
treatments to control aphids and suppress BYD luteovirus were
studied during four seasons in winter wheat in Georgia. Aphids
were mostly the bird cherry-oat aphid and greenbug during vegetative
growth stages and the English grain aphid, Sitobion avenae,
during grain filling. Early plantings had much greater aphid numbers
and incidence of BYDV symptoms than later plantings. Planting
at the end of the recommended period reduced aphid numbers and
BYDV incidence and usually avoided yield losses. Treatment of
seed with imidachloprid (Gaucho) at 0.5 to 1.0 oz ai/100 lb of
seed reduced aphid numbers by ~ 90 % for 45 to 60 days after planting.
Yield responses to Gaucho use varied with aphid number and BYDV
incidence but averaged 56 bu/acre over all trials. A foliar application
of lambda cyhalothrin (Karate 1E/Warrior T) on seedling or tillering
wheat also controlled aphids, suppressed BYDV incidence, and prevented
yield losses. Foliar applications of dimethoate reduced aphid
numbers but did not reliably reduce BYDV incidence or prevent
yield losses. Yield responses were highly correlated with BYDV
incidence (r = 0.96) and aphid numbers (r = 0.58) during the first
60 days after planting. Economic thresholds are three aphids/30-cm
row in the first 30 days after planting and five aphids/30-cm
row at 30 to 60 days after planting. These results show that use
of insecticides to control aphid vectors can effectively suppress
BYDV incidence and minimize grain yield losses in winter wheat.
The cereal leaf beetle, Oulema melanopus, was first discovered
in the late 1980s and now occurs throughout the northern two-thirds
of Georgia and Alabama. Larvae can cause extensive defoliation
during grain heading in winter wheat, and populations have increased
to levels requiring control in the piedmont and mountain regions.
Cereal leaf beetle cannot be reliably controlled by manipulating
planting date or variety maturity, and plant resistance is not
available in southern wheat varieties. Studies in both states
during the last 4 years have examined the impact of larvae infestations
on wheat yield and have examined the efficacy of controlling infestations
using foliarly applied insecticides when populations exceed an
economic threshold. Averaged over all trials, yield reductions
have ranged from 0 to 8+ bu/acre. Most labeled insecticides are
effective at killing larvae at low rates. However, M-trak, which
is a Bacillus thuringiensis product, did not effectively
control larvae. Nevertheless, only lambda cyhalothrin (Karate
1E/Warrior T) consistently provided season-long control with a
single application when applied during egg laying before final
egg hatch. Lambda cyhalothrin also controls aphids, and because
it can be applied during egg laying, often can be tank mixed with
foliar fungicides for leaf rust suppression. Treatment is justified
when populations exceed 0.5 eggs and/or larvae per stem. Several
parasites also are being released in both states to provide long-term
biological control of cereal leaf beetle. The egg parasite Anaphes
flavipes has been released at a number of sites in each state,
and field nurseries have been established in each state to rear
and release the larval parasite Tetrastichus julis. Cereal
leaf beetle will continue to spread southward and increase in
importance as a pest of winter wheat in Georgia and Alabama.
Publications.
Cunfer BM. 1998. Seasonal availability of inoculum of Stagonospora
nodorum in the field in the southeastern U.S. Cereal Res Commun
26:259-263.
Cunfer BM and Castlebury LA. 1998. Smut fungi associated with
ryegrass in wheat fields in the southeastern U.S.and their relation
to Karnal bunt. Phytopathology 88:S1920.
Ge Y-F, Johnson JW, Roberts JJ, and Rajaram S. 1998. Temperature
and resistance gene interactions in expression of resistance to
Blumeria graminis f.sp. tritici. Euphytica 99:103-109.
Johnson JW, Ge Y-F, Cunfer BM, and Barnett RD. 1998. Adult plant
resistance to powdery mildew in wheat. In: Proc 9th Inter
Wheat Genet Symp (Slinkard AE ed). University Extersion Press,
Saskatoon, Sasketchewan, Canada. 3:279-281.
Johnson JW, Buntin GD, Cunfer BM, Roberts JJ, and Bland DE. 1998.
Registration of `GA-Luttrell' barley. Crop Sci 38:1715.
Riessen Van HW, Mask PL, Murphy JF, Cunfer BM, Buntin GD, and
Johnson JW. 1998. Field resistance to the barley yellow dwarf
virus-aphid complex among wheat varieties in Alabama. Phytopathology
88:S91
Shi, A.N., S. Leath, J.W. Johnson, and J.P. Murphy. 1998. Identification
of powdery mildew resistance in Ae. tauschii-derived lines.
In: Proc 9th Inter Wheat Genet Symp (Slinkard AE ed). University
Extersion Press, Saskatoon, Sasketchewan, Canada. 3:317-319.