Properties of the tan spot necrosis toxin.
Cultivar-specific toxic metabolites of Pyrenophora
tritici-repentis are involved in the appearance of necrotic
and chlorotic foliar lesions characteristic of wheat tan spot.
A necrosis-inducing proteinaceous toxin was purified
from isolate 86-124 of P. tritici-repentis,
sequenced by gas phase amino-acid microsequencing, and characterized
by circular dichroism (CD) spectroscopy and isoelectric focusing.
The purified protein had a similar amino acid composition and
molecular weight as previously reported. A sequence of 101 continuous
amino acids was obtained from overlapping proteolytic fragments,
but the amino terminus was blocked and 9 to 16 amino acids could
not be sequenced. A secondary structure prediction based on the
amino acid sequence indicates a beta sheet protein with little
alpha helix, which is in agreement with the structure determined
by CD spectroscopy. Sequence analysis indicated the presence
of a possible membrane adhesion site and several possible phosphorylation
sites that may be involved in phytotoxicity. Additional pathogen
isolates are presently being assessed for symptomology on wheat
differentials to find candidates for biochemical analyses. (Meinhardt
and Zhang)
An electrolyte leakage assay for tan spot necrosis.
Transmission electron microscopy demonstrated that
leaf tissue from toxin-sensitive wheat quickly suffered membrane
degradation after exposure to necrosis toxin. In an effort to
develop a more rapid and quantitative bioassay for necrosis toxin
activity, we found that the conductivity of leaching solutions
increased for at least 8 h after toxin exposures of 4 h or more
in toxin-sensitive wheat, but not in water controls or insensitive
genotypes. Electrolyte leakage from sensitive wheat increased
with toxin concentration between 1 and 20 µg/ml, the highest
concentration tested. This bioassay, which can be completed in
1 day, should facilitate studies of necrosis toxin mode of action.
(Rasmussen, Kwon, and Freeman)
The epidemiology of wheat diseases, with emphasis
on tan spot and Fusarium head blight, is being studied with several
approaches. Greenhouse-grown plants set out in a wheat field
for 24 h show discrete foliar infection periods, so temporally
continuous environmental variables can be analyzed. Similarly,
Fusarium inoculum can be enumerated by washing wheat heads
after a 24 h exposure. A common protocol is being followed in
nine locations in North America to help ensure measurement of
inoculum in a head blight epidemic. In an effort to improve disease
warning systems, an artificial neural network was developed to
distinguish tan spot infection periods. The model used environmental
data as input and was 85 % accurate in a validation test. In
comparison, a stepwise logistic regression model was 69 % correct,
and a multivariate discriminant model was 50 % correct. A second
neural-network model was developed to predict wheat leaf wetness
status under various conditions of input information and crop
density. Predictions of leaf wetness in a validation data set
were 82 to 91 % accurate and duration estimates were correct within
an average of 0.6 to 1.9 h. The accuracy of this model was equivalent
to that of other models that predict dew period, but the model
also can account for leaf wetness caused by rainfall. (Francl,
Panigrahi, Jordahl, and De Wolf)
Conidiogenesis of Pyrenophora tritici-repentis.
Conidiogenesis of P. tritici-repentis
occurs in culture under alternating light and dark cycles with
conidiophores formed in light and conidia in dark. Leaf area
of the tan spot-susceptible spring wheat line `ND495'
was measured nondestructively, and plants were inoculated with
isolate Pti2. After a 24-h wet period, plants were held in a
growth chamber for 10 days. Plants then were subjected to various
light and wetness regimes lasting from 12 to 96 h. Percent disease
severity was estimated at the end of each treatment, and conidia/sq
mm of diseased area were estimated from leaves blended in water
(Riaz et al. Phytopath 81:1298-1302). Conidia failed
to form prior to treatment or after 24 to 96 h in a continuous
light, continuous wet environment. However, unlike the diurnal
requirement for conidiogenesis in vitro, conidia formed after
12 h or longer in the dark. After 96 h of alternating 8 h dark,
wet and 16 h light, dry cycles, 1.7 conidia/mm were produced.
Thus, conidiophores can be initiated on wheat leaves in an unsaturated
atmosphere in the light and a wet period during darkness is sufficient
for conidiogenesis. (Francl, Jordahl, and De Wolf)
Publications.
Francl LJ and Jordahl JG. 1994. Refined procedures
for inoculating wheat seedlings and rating their reaction to Pyrenophora
tritici-repentis. Plant Dis 78:745-748.
Francl LJ. 1995. Challenge of bioassay plants in
a monitored outdoor environment. Can J Plant Path 17:138-143.
Francl LJ, DeWolf E, and Jordahl JG. 1995. Conidiation
of Pyrenophora tritici-repentis. Phytopath 85:1044.
Francl LJ, Panigrahi S, and Padhi T. 1995. Neural
network models that predict leaf wetness. Phytopath 85:1128.
Freeman T, Rasmussen J, Francl L, and Meinhardt S.
1995. Wheat necrosis induced by Pyrenophora tritici-repentis
toxin. In: Proc Microscopy Soc of America. Jones and
Begell Publ, NY. Pp. 990-991
Meinhardt SW, Zhang H-F, Jordahl JG, and Francl LJ.
1995. Characterization of the necrosis toxin of Pyrenophora
tritici-repentis strain 86-124. Phytopath 85:1199.
Rasmussen JB, Francl LJ, Meinhardt S, Kwon C, and
Freeman TP. 1995. Effects of the necrosis toxin produced by
Pyrenophora tritici-repentis on the membranes of wheat.
Phytopath 85:1161.
THE OHIO STATE UNIVERSITY
Ohio Agricultural Research and Development Center, Wooster, OH
44691, USA.
Kim G. Campbell, Robert W. Gooding, David M. Jordan, Larry D. Herald, Richard Minyo, Dimuth Siritunga, and Emily Stowe (Department of Horticulture and Crop Science); Patrick Lipps and A.L Johnston (Department of Plant Pathology); and P. Finney, C. Gaines, D. Gualberto, C. Bergman, J. Donelson, G. Bains, L. Andrews, B. Renner, A. Bugaj, J. Kinney, S. Croskey, T. Andrews, L. Fleege, and M. Raeker (USDA-ARS Soft Wheat Quality Laboratory).
Growing conditions and production in Ohio, 1994-95.
Compiled by R.W. Gooding.
Growing Conditions. The
winter of 1994-95 began with mild weather and ample precipitation.
Moderate temperatures delayed the onset of dormancy until later
than normal. Precipitation amounts increased through January
and February, affording the crop adequate snowfall for insulation.
The season's coldest weather was in February with temperatures
as low as -7_F reported at some locations in Ohio. With the onset
of spring, stands were found to be generally healthy throughout
the state.
Cool, wet conditions during the spring contributed
to high levels of wheat yellow mosaic virus in susceptible wheat
lines. Wet conditions during flowering resulted in severe head
scab throughout the wheat growing regions of Ohio in 1995. Wheat
planted in corn stubble was found to be particularly at risk.
Wheat harvest began during the first week of July.
Because of high temperatures, humidity, and generally favorable
working conditions, the harvest was essentially completed in 4
weeks.
Production. Wheat production
in Ohio increased by nearly 8 % compared to 1994 levels. Harvested
acres of 1.2 million combined with an average yield of 61 bu/acre
resulted in an overall production of 73.8 million bushels of wheat
in 1995. In early June, the estimated statewide wheat yield was
70 bu/acre. Following harvest, the average yield was recorded
to be 61 bu/acre. On a statewide basis, the estimated yield loss
was 11 million bushels. Added to this loss were the dockage most
farmers received at the elevators for low test weights (45-57
lb/bu) and shriveled grain. Most reports indicated a $0.30/bu
dockage was common. Additionally, certified seed producers cleaned
out over 30 % of the seed to achieve acceptable germination standards
(30 % loss on 1.5 million bushels of seed wheat). Thus, $43 million
was lost in production (at $3.90/bu price), dockage amounted to
$22 million, and loss of seed wheat production was $3.8 million
. The cost to the millers and bakers from mycotoxin contamination
is still unknown.
USDA ARS Soft Wheat Quality Lab.
Research. We completed
study of milling-, biochemical-, Mixograph-, gliadin and glutenin
sub unit-, and sugar snap cookie-traits for a second year and
straight dough bread traits to associate with RFLP markers for
78 lines of a `hard x soft' wheat cross. We continued
density studies of wheat to compare with other flour milling and
end-use properties. We also continued research using the NIRS-6500
to determine its potential to predict important flour milling
and end-use properties. We have determined that grain shrivelling
could be determined by instrument. It was found that the Single
Kernel Hardness Tester did not accurately reflect relative kernel
`hardness' when measuring kernels at various moisture
levels, or for various degrees of kernel shrivelling. Also, we
demonstrated that SKHT raw data can relate highly to kernel softness
by milling. We developed formula substitution for chlorination
of flour in cake and other related products. Modification studies
of a flour hydration procedure to make it more sensitive to low
levels of field sprouting were continued.
Evaluation. Evaluation
of 7,950 micro-, 1600 advanced-, and about 300 drill-strip nursery
samples from 18 public and 12 private breeding programs was completed.
We installed movable-tier shelving for the second of three rooms.
We completed a fourth year of evaluating uniform nurseries from
sub-location composites. Statistical treatment will now be completed
cooperatively with geneticists.
Ohio Soft Red Winter Wheat Performance Test.
Ohio is a leading state in the production of soft
red winter wheat. The selection of superior cultivars has the
potential to add millions of dollars in revenue to farmers'
income. The Ohio State University annually tests public and private
wheat varieties under field conditions to provide information
on the comparative performance of cultivar traits. Field studies
are conducted across several Ohio environments to compare agronomic
performance on yield, test weight, heading date, plant height,
lodging, and diseases present. In cooperation with the USDA-ARS
Soft Wheat Quality Laboratory, grain quality evaluations provide
information on milling and baking properties.
Table 1. Reaction of soft red winter wheat varieties to disease in Ohio, revised 1995.
__________________________________________________________________________________________
Septoria nodoru m Wheat
Powdery Leaf leaf glume yellow
Brand Variety mildew rust blotch blotch mosaic
__________________________________________________________________________________________
AGRA GR863 MR MR S S MR
GR876 MR R MR R MR
GR915 R MR R R R
GR933 S MS MS MS R
GR942 MR S S S S
Agripro Twain MR R S S ó
Sawyer MS MR MS MR S
Pontiac S S S S S
Clemens S R MS MR S
Certified Becker VS S MS S MR
Caldwell S MS S VS S
Cardinal S S MR MR MR
Clark MS S S S MR
Dynasty MS S S S R
Excel S S S S MR
Freedom MR R MR MR MR
Glory MR S MS MS R
Grant S MR S S MR
Hopewell MR S MS MS R
Jackson MR S MR MR S
Madison MR MS MS MS R
Wakefield MR S MR MS MS
Countrymark 544 R MR MS S MS
558 R MS MS MS S
568 MR S S MS MR
Greenland GL9240 S ó ó ó S
GL9400 MR R MS S MS
Hytest Succession R MR S MS S
Nosco Classic RW151 MR MR S MS MS
Table 1 (continued). Reaction of soft red winter wheat varieties to disease in Ohio, revised 1995.
__________________________________________________________________________________________
Septoria nodoru m Wheat
Powdery Leaf leaf glume yellow
Brand Variety mildew rust blotch blotch mosaic
__________________________________________________________________________________________
Pioneer 2510 MS R MR MR R
2545 MR S S S R
2548 MR MS S MS S
2550 S R MS MS MR
2552 R S MR MS R
2555 S MS MR MR ó
2571 MR R S S S
Rupp RS927 S S S S S
Shur Grow SG1550 MS MS MS MS S
Steyer Hoppes S S MS MS MR
Kline R MR MR MR R
McLane R MS MR MR S
Neuleib S MS MS ó ó
Podach MS MS S MS S
Rowland R R S MR R
Stine 501 R MR R R MR
Terra SR204 MR MR MS MS MS
SR205 S MS MS MS R
Thompson TS4020 R MR S MS S
TS5020 R S S MS S
Voris V8040 S S MS MS R
Wellman W9140 MS MR MR ó ó
W9350 MS MS S MS S
W9420 MR S S MS MR
W9540 R S MS MS R
__________________________________________________________________________________________
R = resistant; MR = moderately resistant; MS = moderately
susceptible; S = susceptible; and VS = very susceptible; ó
= no data available. Data obtained from disease ratings during
1993, 1994, and/or 1995. Changes in resistance or susceptibility
from one year to the next are expected because of the presence
of, absence of, or change in races of pathogens.