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2008 Southern Division
Meeting Abstracts
(Joint with the Southern Association of Agricultural Scientists (SAAS)
February 3-4, 2008 - Dallas, Texas
Posted online April 9, 2008
Comparative epidemiology of Pierce’s disease in grape varieties in Texas.
D. N. APPEL and C. P. Torres. Dept. of Plant Pathology and Microbiology,
Texas A&M University, College Station, TX 77845.
Pierce’s disease, caused by Xylella fastidiosa, is a limiting factor
for wine grape production in Central Texas. Annual surveys were conducted
during 2002–2006 in affected vineyards to compare disease progress
(apparent infection rate = r) in commonly planted grape varieties. In
Vineyard A the highest rate of disease progress, as measured by mortality
among 5 varieties, was in Cabernet Sauvignon (r = 0.79, n = 1627 vines). The
other varieties in Vineyard A were Pinot Grigio (r = 0.59, n = 3477), Merlot
(r = 0.56, n = 1419), Chardonnay (r = 0.27, n = 3267), Cabernet Franc (r =
0.18, n = 557), and another block of Cabernet Sauvignon (r = 0.07, n =
1111). At Vineyard B, mortality rates were consistently higher. Among 5
varieties, the highest rates were found in the Merlot (n = 868) and Cabernet
Sauvignon (n = 1124) grapevines (r = 1.7 and 1.69, respectively). An edge
effect was occasionally, but not consistently, observed in the vineyards.
According to ordinary runs analysis, pathogen spread proceeds more rapidly
along rows than across rows, but the effect is most pronounced early in the
epidemic. No consistent trends in disease progress were detected among the
varieties.
Evaluation of day versus night fungicide sprays for the control of peanut
diseases. J. AUGUSTO (1), T. B. Brenneman (1), P. Sumner (2), A. K.
Culbreath (1), and A. S. Csinos (1). (1) Dept. Plant Pathology and (2) Dept.
Biological and Agricultural Engineering, University of Georgia, Tifton, GA
31793.
Tebuconazole (0.53 kg/ha a.i., 4 applications) and azoxystrobin (1.34 kg/ha
a.i., 2 applications) were sprayed on peanut during the day or at night when
the leaves were folded to compare disease control and yield. Two experiments
were conducted in 2007 with the cultivar Georgia Green in 2-row plots with 6
replications. Night and day sprays of both fungicides provided similar
control of early leaf spot (Cercospora arachidicola), but night
sprays reduced southern stem rot (Sclerotium rolfsii) incidence by
61% compared to day sprays. Although day sprays of both fungicides decreased
southern stem rot compared to the control, neither one significantly
increased pod yields. Night sprays of azoxystrobin and tebuconazole
increased yield by 1752 kg/ha and 944 kg/ha, respectively, compared to the
same treatments applied during the day. These results suggest that night
sprays can dramatically increase fungicide efficacy on southern stem rot and
increase peanut yield.
Infection of two Ipomoea spp. by Sclerotinia minor. M. A.
BATLA (1), J. E. Woodward, P. A. Dotray (1), and T. A. Baughman (2). (1)
Dept. of Plant & Soil Science, Texas Tech University, Lubbock, TX 79409; (2)
Dept. of Soil & Crop Sciences, Texas A&M University, Vernon, TX 76384.
Sclerotinia blight, caused by Sclerotinia minor, is an important
disease of peanut in Texas. The pathogen has been isolated from naturally
infected Ipomea spp.; however, the impact of these findings on
commercial production is uncertain. In October 2007, a field known to be
infested with S. minor was randomly sampled to determine the
distribution and infection frequency of I. hederacea and I.
coccinea. Sclerotinia minor sclerotia were enumerated from
symptomatic plants. I. hederacea was the predominate species in the
area comprising 65% of the weeds sampled. Infection frequencies were 31 and
20% for I. hederacea and I. coccinea, respectively. Sclerotia
production was two-fold greater on I. hederacea than I. coccinea.
These results indicate that both I. hederacea and I. coccinea
can serve as hosts for, and may impact inoculum densities of S. minor.
“Going against the grain,” maize 13-lipoxygenase mutants are more resistant
to Aspergillus flavus and Colletotrichum graminicola. S.
A. CHRISTENSEN and M. V. Kolomiets. Dept. of Plant Pathology and
Microbiology, Texas A&M University, College Station, TX 77843.
Maize oxylipins (oxygenated fatty acids), generated by the lipoxygenase
(LOX) pathway, have been accredited for their roles in plant development and
defense response to insects, pests, and pathogens. A recent study showed
that 9-LOX derived maize oxylipins are structurally similar to fungal
oxylipins and may be hijacked by the fungus and used for conidiation and
mycotoxin biosynthesis. To date, 13-LOX products (i.e. jasmonic acid and
green leafy volatiles) have largely been characterized as defense-related
compounds against various bacteria, fungi, pests and insects. Here, we
generated knock-out mutants in the maize 13-LOX, ZmLOX10, to show
that specific fungi may have evolved to use 13-LOX products in order to
colonize the host and synthesize mycotoxins. This hypothesis is prompted by
our finding that lox10 mutants, deficient in green leafy volatiles,
are more resistant to Aspergillus flavus and Colletotrichum
graminicola.
Research and extension activities monitoring the health of the urban forest
in New Orleans, Louisiana two years post-hurricane Katrina. D. J.
COLLINS, Y. Qi, K. Abdollahi, Z. Ning, J. Preuett, F. Namwamba, and A.
Johnson. Urban Forestry Program, Southern University College of Agricultural
Family and Consumer Sciences, Baton Rouge, LA 70813.
Hurricane Katrina was one of the most catastrophic hurricanes in the history
of the United States. A majority of the City of New Orleans, Louisiana was
flooded with up to 15 to 20 feet of water for 6 weeks after the levees
failed when the hurricane moved in land. This is a update of ongoing
research and extension activities monitoring the health of the urban forest
in New Orleans two years post-hurricane Katrina. Preliminary studies have
shown tree such as live oaks (Quercus virginiana) and Palm species to
have survived the prolonged flooding. We are educating community groups
concerning tree health care, and using state-of-the-art technology such as
the PiCUS SONIC Tomography to detect internal decay in trees. Plans are
underway to monitor long-term the health of key urban forest areas in New
Orleans such as Lakeview, City Park, and the Lower Ninth Ward.
Cultivar and fungicide effects on Pythium leak of snap bean. J. P.
DAMICONE (1), J. D. Dominiak-Olson (1), and B. A. Kahn (2). (1) Dept. of
Ento. & Plant Path.; (2) Dept. of Hort. & L.A., Oklahoma State Univ.,
Stillwater, OK 74078.
Pythium leak of snap bean, caused primarily by P. aphanidermatum and
P. ultimum, causes pod decay and is a problem in processing crops
where pods are harvested in bulk. Fungicide programs alone have not provided
adequate disease control. The objective was to evaluate snap bean cultivars
for their reaction to Pythium leak with and without fungicide (mefenoxam +
copper hydroxide) treatment. Entries were compared to the regional standards
of Roma II for flat pod types and Nelson for round pod types. Over two
years, disease incidence (DI = plants with symptoms) in untreated plots
averaged 22% for Roma II and 30% for Nelson. Among the flat-pod types,
entries that had less (P < 0.05) disease and yields similar to or
greater than Roma II included Bogota (DI = 13%), Navarro (DI = 12%), Romano
942 (DI = 12%), Cerler (DI = 10%), Tapia (DI = 10%), Primo (DI = 10%), and
Ebro (DI = 5%). Among the round pod types, PLS 75 (DI = 14%) and SB 4261
(11%) had less disease than Nelson. However, all of the entries with round
pods yielded less than Nelson. Over entries, fungicide treatment reduced
disease incidence by 40% and did not increase yields.
Persistence of Phytophthora blight in the Texas High Plains. R. D.
FRENCH-MONAR (1) and T. Isakeit (2). (1) Dept. of Plant Path. & Microbiol,
Texas A&M AREC, Amarillo, TX 79106; (2) Dept. of Plant Path. & Microbiol.,
Texas A&M University, College Station, TX 77843.
Phytophthora blight of pepper and cucurbits, caused by Phytophthora
capsici, has increased in importance in many production areas of the
United States. In 2006, the first report of this disease on pumpkin and
winter squash in Texas was reported in Yoakum County, Texas High Plains. In
the summer-fall 2007 season, this disease was observed in adjacent Gaines
County on chile pepper production. For both years, unusual frequent showers
and higher-than-average precipitation was present. Isolates recovered from
2006 and tested were of the A2 compatibility type and sensitive to mefenoxam
at 5 ppm. Nine isolates recovered from infected roots in 2007 were found be
sensitive to mefenoxam. Six isolates were of the A1 compatibility type and
three were of the A2 compatibility type. After three days, colony diameter
growth response at 25°C on 20% V8 juice agar for isolates recovered from
2006 and 2007 averaged 35.5 and 45.4 mm, respectively. The potential for
natural oospore production and pathogen survival in both soil and weed host,
could help establish this pathogen in vegetable production in the Texas High
Plains and beyond. Further characterization of the pathogen, its biology,
and ecology will allow for better plant disease management strategies.
Distribution and stability of a new race of Phytophthora nicotianae,
the causal agent of black shank of tobacco, in NC. C. A. GALLUP, K. L.
Ivors, and H. D. Shew. Plant Pathology, NC State University, Raleigh, NC
27695.
The black shank pathogen occurs in all tobacco-producing areas of NC. Races
are delimited based on their ability to overcome two host resistance genes,
Phl from Nicotiana longiflora and Php from N.
plumbaginifolia. Race 1 increases by selection from predominant race 0
populations in response to deployment of Phl and Php
single-gene resistance. A new race, race 3, was recovered in NC in recent
years. Race 3 isolates cause disease on varieties with the Phl gene,
but not the Php gene. A multiyear survey is underway to determine the
distribution of races in NC. In 2006, 575 isolates were collected from 65
fields in 23 counties. Races 0, 1, and 3 were recovered across NC, with race
1 predominant, correlating with the deployment of the Php gene. Race
3 was characterized phenotypically and genetically for stability. Based on
greenhouse inoculation trials, race 3 is unstable, as confirmed by the
recovery of race 0 from race 3-infested soil. Variability among asexual
progeny in race 3 was further investigated using AFLPs.
The influence of brown rust (Puccinia melanocephala) on sugarcane
yield. C. A. HOLLIER and J. W. Hoy. Department of Plant Pathology and
Crop Physiology, LSU Agricultural Center, Baton Rouge, LA.
Brown rust, caused by Puccinia melanocephala, is a major disease in
sugarcane in many areas of the world. Yield losses have been estimated at
10–20 percent under good growing conditions and up to 50 percent where
growing conditions were poor or the disease was unusually severe. In
Louisiana, brown rust was first detected in 1970’s and could be found
annually at varying levels of severity. It was not until 2000 that the
disease was considered yield limiting, but losses due to brown rust had not
been measured therefore yield loss field studies were implemented to
determine the impact of common rust on the local crop. Fungicide treatments
controlled brown rust each year of the three-year study. The amount of yield
loss was affected by the time of occurrence and duration of the epidemic.
Significant yield losses were demonstrated when rust was controlled
throughout the epidemic period or during mid- and late-epidemic. Yield
losses relative to the “disease-free” check over the three-year study were
17 percent in cane tonnage and 18 percent in sucrose production. Yield
reduction was due to stalk weight and a slight stalk population decrease.
Determination of the optimal temperature for infection of creeping bentgrass
roots by Pythium volutum. J. P. KERNS and L. P. Tredway. Dept. of
Plant Pathology, NCSU, Raleigh, NC 27695.
Symptoms of Pythium root dysfunction (PRD) in creeping bentgrass are most
common in the summer during periods of heat and drought stress. However, our
observations indicate that Pythium volutum, a causal agent of PRD, is
most active during the fall and spring. To determine the optimal temperature
range for infection by P. volutum, ‘A-1’ creeping bentgrass was
seeded into cone-tainers containing sand meeting USGA specifications and
placed in the greenhouse. Eight weeks after seeding, plants were inoculated
with one of 5 P. volutum isolates. After inoculation, the
cone-tainers were transferred to growth chambers at constant 12°C, 16°C,
20°C, or 24°C. After 4 weeks, the temperature in all chambers was increased
to 32°C/26°C to induce foliar symptoms. Symptoms typical of PRD developed two
weeks after increasing the temperature. Disease severity was greatest at
16°C, 20°C, and 24°C. Reductions in root depth were not observed prior to
raising the temperature to 32°C/26°C. However once the temperature was
elevated, root dieback rapidly occurred. These results demonstrate that
P. volutum is most active at temperatures prevalent during the fall and
spring in NC.
Effect of the surfactant “Affinity™” on the delivery of a biological
nematicide, Paecilomyces lilacinus, into the soil profile. J. E.
McGehee and K. L. ONG. Department of Plant Pathology and Microbiology, Texas
A&M University, TX.
The ineffectiveness of Paecilomyces lilacinus strain 251 (Melocon WG)
as a biological nematicide in an initial evaluation on golf greens resulted
in our finding that the fungal spores of this biological agent were not
penetrating sufficiently into potential nematode infection sites. Much of
the product was localized in the thatch layer (upper 1 inch). The objective
of this study was to identify if the surfactant used as a 24-hr pretreatment
and/or in a mix with the biological agent treatment will enhance the
delivery of the P. lilacinus deeper into the soil profile. Seven
combinations of pretreatment and treatment (spraying and drenching) were
evaluated. Distribution of P. lilacinus were enumerated 24 hr and
1-week post treatment. The result that we obtained indicated that none of
our application regimes were sufficient in delivering this biological agent
deeper than 2 inches into a bermudagrass “Champion” golf green.
Distribution of Iris yellow spot virus in Georgia. C. NISCHWITZ,
R. Gitaitis, and S. W. Mullis. University of Georgia, Coastal Plain
Experiment Station, Dept. of Plant Pathology, Tifton, GA 31794.
Iris yellow spot virus (IYSV) was first observed in Georgia in 2003. The
virus has a limited host range and onion, a winter crop, is the only host
grown commercially. The epidemiology of IYSV was studied by collecting weeds
at an onion cull pile with a history of IYSV and tested for the virus using
ELISA and RT-PCR. Spiny sowthistle (Sonchus asper) was confirmed as a
host. A survey of spiny sowthistle was conducted across Georgia to determine
virus distribution within and outside the onion-growing region. Samples were
collected along four transects, using the Vidalia onion-growing zone as the
center of origin. IYSV infected sowthistles were found up to 333 km to the
north and northwest of the Vidalia region, as well as around the experiment
station in Tifton. No positive sowthistles were found south, far southwest
and east of the Vidalia region. Although spiny sowthistle is not prevalent
during the summer, thus it cannot serve as a “green bridge” for the virus to
survive locally between onion crops, we concluded that IYSV has moved into
at least one indigenous host and has the potential to move long distances.
Do foliar applications of fungicides benefit corn production in the
Mid-South? G. B. PADGETT, D. Lanclos, M. A. Purvis, and C. A. Hollier.
Dept. of Plant Pathology and Crop Physiology, Louisiana State University,
Baton Rouge, LA 70803.
Interest in foliar fungicide applications to corn in the Mid-South increased
dramatically during 2007; however, data to support their benefit is limited.
Studies were initiated during 2007 to evaluate the impact of selected
fungicides on disease and plant development. Two fungicides were applied at
tasseling to eight recommended corn varieties, and in other studies several
fungicide treatments were evaluated on a single variety. To document
fungicide efficacy, disease development was monitored during the growing
season in all studies. Stalk and ear weight, as well as stalk density, was
determined for each treatment to assess plant development. Disease epidemics
did not develop to appreciable levels in most tests. Corn yields and stalk
density did not differ among treatments. More research is necessary to
determine if fungicides preserve grain yield and quality in the presence of
disease.
STAMINA™, a new strobilurin seed treatment fungicide that increases
tolerance to low temperature stress. M. C. PAGANI, H. L. Ypema, J. S.
Barnes, and C. Lindholm. BASF Corp., Research Triangle Park, NC 27709.
STAMINA™ (BAS 500 12 F), containing the active ingredient pyraclostrobin, is
a new strobilurin seed treatment fungicide in development by BASF. In
addition to providing a direct antifungal activity, pyraclostrobin has been
found to affect plant physiological processes in a variety of crops.
Experiments were conducted to assess the protective effect of Stamina seed
treatment applications against low temperature damage of seedlings at early
stages. At the rates tested in laboratory, greenhouse and field research
trials, Stamina demonstrated ability to protect seedlings from freeze damage
at certain temperatures. An overview will be presented of this effect of
Stamina on corn, soybean, sugar beet, spring wheat and cotton. On all crops
evaluated, Stamina decreased % seedling mortality at early growth stages
when compared to the non-treated control. Data suggest that in addition to
providing excellent control of major diseases, Stamina could also provide
seed treatment benefits through better cold stress tolerance.
Two closely related lipoxygenase genes have distinct roles in the regulation
of mycotoxin and conidia production by Aspergillus flavus in maize
seed. Y. S. PARK and M. V. Kolomiets. Dept. of Plant Pathology and
Microbiology, Texas A&M University, College Station, TX 77843.
Colonization of kernels by Aspergillus flavus is a major limiting
factor for maize production worldwide because they contaminate seed with
highly aflatoxins. Herein we have reported on the isolation and molecular
and functional characterization of two maize lipoxygenases (LOXs) genes,
ZmLOX4 and ZmLOX5. The two genes are highly related to each other
sharing >90% nucleotide and amino acid sequence identity. Despite this
similarity, the expression of these genes is regulated differentially by
diverse defense hormones, pathogen infection and wounding. Northern blot
analysis demonstrated that ZmLOX4 was induced by jasmonic acid (JA)
and salicylic acid (SA), whereas ZmLOX5 was inducible by JA, abscisic
acid, ethylene, and mechanical wounding, but not SA. To elucidate their
function in maize resistance to contamination with aflatoxin, we generated
Mutator transposable insertional mutants in both genes. Field testing
of knock-out mutants and near-isogenic wild types revealed that lox5
mutants accumulate significantly lower levels of aflatoxin contamination,
whereas lox4 mutants did not show any significant difference in the
aflatoxin levels.
Root development and water use efficiency of hard red winter wheat infected
with Wheat streak mosaic virus. J. A. PRICE and C. M. Rush. Texas
Agricultural Experiment Station, Amarillo, TX 97106.
Greenhouse and field studies were conducted with two varieties of wheat, one
susceptible and one resistant to Wheat streak mosaic virus (WSMV).
During the greenhouse trials, varieties were grown in three different water
regimes of 30, 60, and 80% pot saturation capacity. After inoculation with
WSMV, plants were grown for approximately 4 weeks and then harvested. Root
and shoot weights were taken to examine biomass development of infected
plants. Significant differences (P < 0.0001) were found in
water*inoculation reactions. Root biomass and water use efficiency were
significantly reduced in inoculated susceptible plants. Field studies were
conducted under three different water regimes based on reference
evapotranspiration rates. Significant reductions in forage, grain yield, and
crop water use efficiency were detected in the inoculated susceptible plots,
during the 2005 season. Soil moisture was also significantly higher in
inoculated plots. Results indicated that WSMV caused a significant reduction
in root biomass and water uptake by infected plants.
Phytophthora root rot - A new disease of cabbage in Kentucky. K. W.
SEEBOLD, E. Dixon, and P. Vincelli. Plant Pathology Dept., University of
Kentucky, Lexington, KY 40546.
Production of cabbage in Kentucky is a relatively small industry aimed
historically at the fresh market. In 2007, large-scale production of
processing cabbage was initiated in a 40-acre field in western KY with no
previous history of cruciferous crops. Following a heavy irrigation event,
symptoms were observed on cabbage plants in mid-August that included
wilting, purplish discoloration of lower stems, necrosis of roots and stems,
and disking of the pith of affected stems. Approximately 10% of plants in
the field were affected, with most symptomatic plants being located in areas
where soils had become saturated. Tissue samples were floated in sterile,
distilled water and, based upon morphological characteristics, a species of
Phytophthora was recovered. Pure cultures of the pathogen were used
to obtain DNA, and an amplicon was obtained using ITS4 and ITS6 primer
pairs. The amplicon was sequenced and submitted to GenBank, and was found to
share 99–100% similarity to known sequences of P. drechsleri.
Additionally six-week-old cabbage and cauliflower plants were inoculated
with the suspect pathogen, resulting in symptoms 7–10 days later that were
identical to those observed in the field; a Phytophthora spp. with
identical morphology to the original isolate was recovered from symptomatic
plants. These results confirm the first case of a root rot of cabbage in the
U.S. caused by P. drechsleri.
Baseline sensitivity of Fusicladium effusum to thiophanate-methyl,
dodine, fentin hydroxide, and propiconazole using a microtiter plate assay.
M. SEYRAN, K. L. Stevenson, and T. B. Brenneman. Dept. of Plant Pathology,
University of Georgia, Tifton 31793.
Commercial production of pecans in the southeastern U.S. relies on fungicide
applications to control scab, caused by Fusicladium effusum. To
monitor development of fungicide resistance in the pathogen, baseline
sensitivity profiles to the major scab fungicides were established.
Eighty-nine monoconidial isolates of F. effusum were obtained from
three orchards in Georgia with no known fungicide history. Isolates were
tested for sensitivity to propiconazole, dodine, fentin hydroxide and
thiophanate-methyl using a mycelial growth assay in liquid medium in
microtiter plates. The 50% effective dose EC(50) values ranged from 0.008 to
0.595 mg/L with a median of 0.0484 mg/L for dodine (n = 66), 0.115 to 4.540
mg/L with a median of 0.759 mg/L for fentin hydroxide (n = 45), and 0.0114
to 0.793 mg/L with a median of 0.0614 mg/L for propiconazole (n = 65). All
tested isolates were sensitive to thiophanate-methyl and failed to grow at
1.0 mg/L. These sensitivity data will serve as baselines for future
fungicide sensitivity monitoring in commercial pecan orchards.
Presence of Xylella fastidiosa in Oklahoma. D. L. SMITH, J.
Dominiak-Olson, P. Mulder, and S. vonBroembsen. Dept. Ento. and Plant Path.,
OK State University, Stillwater, OK 74078.
Bacterial leaf scorch (BLS) of ornamental and horticultural crops and
Pierce’s disease (PD) of grape, caused by the bacterium Xylella
fastidiosa, are a significant problem in much of the southern United
States. In 2004, the presence of X. fastidiosa was confirmed for the
first time in Oklahoma in Ulmus Americana. X. fastidiosa was
identified from weeds and trees in 8 of 12 counties surveyed in 2005. DNA
sequencing was used to identify one isolate from Morus sp. that was
closely related to PD strains. In 2003 and 2004, a survey for principal
leafhoppers that transmit PD associated strains of X. fastidiosa was
performed in southern counties of Oklahoma. Several polyphagous,
xylem-feeding leafhopper species were recovered over the two-year period,
but no glassy-winged sharpshooters (primary vector of X. fastidiosa)
were identified. These data suggest that X. fastidiosa can survive
and has potential to be transmitted to many hosts in Oklahoma. Further
studies need to be conducted to determine the full distribution and economic
impact of X. fastidiosa in Oklahoma.
Role of microbial communities on the occurrence of Peanut soil borne
pathogens among different cropping sequences. H. SUDINI, K. Bowen, and
R. Huettel. Dept. of Entomology and Plant Pathology, Auburn University, AL
36849.
In peanut soils, as in any soil, there is a complex biotic structure that
affects plant health, crop productivity and the sustainability of that
productivity. In this study, we are investigating the effect of soil
microbial communities associated with different peanut rotation sequences on
the occurrence of important soil borne pathogens such as Sclerotium
rolfsii (causal agent of stem rot of peanut) and mycotoxigenic
Aspergillus flavus. Microbial communities, which are fingerprinted with
ARISA (Automated Ribosomal Intergenic Spacer Analysis), indicate
similarities among ARISA profiles of the same cropping sequence even though
the replicated plots are widely spaced (165m). This suggests the impact of
cropping pattern on the microbial communities. To further strengthen our
observations, we conducted 16S rRNA survey to identify the divisional level
affiliation of bacteria among different rotation sequences. Research is
underway to determine the occurrence of A. flavus and S. rolfsii
in different crop rotations with respect to the microbial community
fingerprints at various stages of crop growth.
Isolation of Xylella fastidiosa from seven grape varieties in a Texas
vineyard. C. P. TORRES and D. N. Appel. Dept. of Plant Pathology &
Microbiology, Texas A&M University, College Station, TX 77843.
Xylella fastidiosa is a slow growing, xylem-limited bacterium that
causes Pierce’s Disease of grape. In 2007 samples were collected from vines
to obtain isolates in order to evaluate strain relationships within the
vineyard. Samples consisted of petioles harvested from eight grape (Vitis
sp.) varieties growing for 5 years in a vineyard in Brenham, Texas. When
available, symptomatic vines were sampled to maximize represented areas and
improve successful isolation. Blanc du bois was the only variety from which
no isolates were obtained (n = 24). The highest isolation frequency was from
Chambroucin (72.7% from n = 33 attempts). The percentages of successful
isolations for other varieties were as follows: Muscat Blanc 11.6% (n =
108), Merlot 33.3% (n = 38), Cabernet Sauvignon 42.9% (n = 14), Syrah 48.1%
(n = 45), Primitivo 51.5% (n = 44), Ruby Cabernet 56.7% (n = 30), with an
overall of 35.6% (n = 337). There was a 72% rate of isolation for vines
which expressed advanced symptoms and a 51% rate for vines with moderate
symptoms; where as 7% of asymptomatic vines produced successful isolations.
The isolation of cultures from asymptomatic vines demonstrates the need for
a tiered diagnostic protocol when evaluating Pierce’s Disease within a
vineyard.
Infection of sesame seed by Alternaria tenuissima. J. E. WOODWARD
(1), C. Nu (1), R. J. Wright (1), and B. M. Pryor (2). (1) Dept. of Plant &
Soil Science, Texas Tech University, Lubbock, TX 79409; (2) Dept of Plant
Sciences, University of Arizona, Tucson, AZ 85721.
Alternaria leaf spot (ALS), caused by Alternaria sesame, is an
economically important disease of sesame (Sesamum indicum) throughout
the world. In the Texas High Plains, a small spore-type Alternaria
sp. has been associated with ALS symptoms. Seed capsules from plants
exhibiting ALS symptoms were collected from two production fields and
assayed for transmission of Alternaria spp. Seed (n = 1000) from each
location were surface disinfested and plated on a selective medium.
Infection levels ranged from 55 to 63% from the two locations. Cultures were
olive-green with a dark center, identical to those obtained from symptomatic
foliage. Morphological characteristics of conidia and analysis of fungal DNA
implicated A. tenuissima as being the causal agent. These results
suggest that Alternaria spp. other than A. sesame are capable
of infecting sesame seed, thus serving as additional sources of inoculum for
ALS epidemics.
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