Fungicide-treated Plots Research Articles

Simulated Rainfall and Mowing Impact Fungicide Performance When Targeting Dollar Spot in Creeping Bentgrass.

In this 2-year field study, four chemically diverse fungicides (i.e., chlorothalonil, boscalid, iprodione, and propiconazole) were evaluated for their ability to control dollar spot (Sclerotinia homoeocarpa) in creeping bentgrass (Agrostis stolonifera) as affected by simulated rain and mowing timing. Simulated rain (25 to 32 mm) was imposed about 30 min after fungicide application and was compared to rain-free plots. One set of plots was mowed in the morning when the canopy was wet with dew and compared to plots that were mowed when the canopy was dry in the afternoon. The percent reduction in dollar spot control associated with simulated-rain versus rain-free treatments in 2007 and 2008, respectively, was as follows: chlorothalonil 67 and 83%; propiconazole 42 and 79%; boscalid 48 and 70%; and iprodione 33 and 66%. When disease pressure was low, all fungicides subjected to simulated rain provided effective dollar spot control for 7 or more days following the initial application in each year. Across all fungicide-treated plots over 2 years, the average percent reduction in dollar spot associated with morning mowing ranged from 54 to 65%. The reduction in dollar spot severity in morning-mowed plots improved the performance of all fungicides.

Efficacy of fungicide mixtures for the management of Phytophthora infestans (US-1) on potato

Fungicide application strategies (timing, frequency, rates and mixtures) are important for the control and resistance management of potato late blight caused by Phytophthora infestans. The efficacy of fungicide mixtures consisting of fenamidone + mancozeb and propamocarb HCL + mancozeb at various rates and in spray regimes containing metalaxyl and mancozeb was evaluated for late blight control (US-1) at four locations in Kenya. Propamocarb HCL + mancozeb significantly (P < 0.05) reduced foliar blight compared with mancozeb and the untreated control under moderate to severe disease pressure. Disease severity was significantly lower following application of propamocarb HCL + mancozeb at a rate of 4L ha-1 than at rates of 2L and 3L ha-1 in 1999 and 2000, but it was not significantly lower following applications at a rate of 3L ha-1 in 2000 and 2001. There were no significant differences in mean final late blight score among the three rates of 0.9, 1.0 and 1.1 kg ha-1 of fenamidone + mancozeb. All fungicide mixtures and application sequences significantly reduced the area under the disease progress curve and final late blight scores as compared with the unprotected control. Total and marketable tuber yield significantly (P < 0.05) increased in all fungicide-treated plots.

Non-target impacts of soybean rust fungicides on the fungal entomopathogens of soybean aphid

Soybean aphid, Aphis glycines, has caused serious economic damage to soybean across the North Central US since its introduction to North America in 2000. The management of another invasive soybean pest, Asian soybean rust, Phakopsora pachyrhizi, using foliar fungicide applications has the potential to impact soybean aphid populations by suppressing beneficial fungal entomopathogens. In 2005 and 2006, we applied recommended soybean rust fungicide treatments, consisting of strobilurin and triazole fungicides, to small soybean plots in two locations to assess if such applications might suppress aphid fungal epizootics. In Lamberton, MN, in 2005, during the epizootic, fungicide-treated plots averaged 2.0 ± 0.7% (mean ± SE) disease prevalence while untreated plots averaged 14.2 ± 5.6%. In 2007, we applied strobilurin and strobilurin–triazole mix fungicides to single-plant microplots either before or after release of Pandora neoaphidis, the most commonly observed aphid pathogen in 2005 and 2006. Treatments that contained a mixture of two active ingredients significantly lowered peak and cumulative aphid disease prevalence in both early and late reproductive stage soybeans indicating that fungicide mixtures used to manage soybean rust can negatively impact an aphid-specific fungal pathogen. However, no consistent soybean aphid population response was observed in these studies of low levels of aphid fungal infection.

The economics of fungicide use in winter wheat in southern Sweden

In southern Sweden, fungicide treatment of winter wheat is prevalent and recommended almost routinely against leaf blotch diseases. However, yield increases and hence the resulting net returns from fungicide use are highly variable within and between years. These variations raise questions about whether, when and how fungicides should be used. To help answer these questions, a thorough economic evaluation of fungicide use was carried out, based on results from untreated plots and fungicide-treated plots in trials in farmers' fields, 1983–2007. Scenarios with varying grain prices and costs of fungicide treatment were evaluated and examined. Doubling and tripling the grain price led to the largest impact on the net return from fungicide treatment, followed by increasing cost of the fungicide. Other costs were of minor importance. The mean net return from fungicide use was no more than 12 € ha −1 over the 25 years (2008 grain prices and costs used in calculations). Furthermore, the mean net return was negative in 10 years and less than 50% of the entries were profitable to treat in 11 years. Changes over time and changes in controllable factors (e.g. fungicide and cultivar choice, crop rotation, techniques) and uncontrollable factors (e.g. emerging and new diseases, price relations) influenced the profitability of fungicide use. Fungicide use was in fact more profitable (mean net return 21 compared with 3 € ha −1) during the latter part of the period (1995–2007) than in the earlier part (1983–1994). Improved decision support systems in a holistic framework based on sound economics are urgently needed.

Response of Peanut to Fungicide and Phosphorus in On-station and On-farm Tests in Ghana

Abstract Peanut (Arachis hypogaea L.) is an important component of cropping systems in West Africa. Identifying production constraints in farmers' fields and evaluating possible management strategies are of prime importance to improve peanut productivity and adoption of new technologies. The objective of our research was to study the influence of fungicides and phosphorus application on severity of leaf spot, dry matter production and pod yield of peanut crops grown in on-station and farmer participatory tests (on-farm conditions) in Northern Ghana. On-station tests to evaluate yield benefits of fungicide sprays and applications of phosphorus were conducted at Wa. On-station tests included two fungicide treatments (no-spray versus fungicide spray) at four P fertilizer levels (0, 30, 60 and 90 kg P ha−1). On-farm tests were conducted in three villages Nakor, Piisi and Janguasi with participation of 6–11 farmers per village. On-farm tests included three treatments: (i) farmers' practice of no-fungicide and no-fertilizer (control), (ii) only fungicide, and (iii) combination of fungicide and phosphorus. The commonly grown Spanish type cultivar Chinese (90-d duration) was selected. Both leaf spot and lack of phosphorus nutrition were yield-limiting factors in on-farm tests. Applications of fungicide were effective in controlling leaf spot and improved peanut pod yield on average by 49% in the three tested field sites in on-farm tests and by 40% in on-station tests. Application of phosphorus to fungicide-treated plots further increased pod yield by 32% when compared to fungicide alone in on-farm tests. Combination of both fungicide and P fertilizer improved peanut pod yield by 95% (ranged from 75 to 120%), when compared to farmers' practice of no-fungicide and no-fertilizer.

Yield and disease control in winter wheat in southern Sweden during 1977–2005

Fungicides are routinely used to prevent yield losses in winter wheat in southern Sweden. Yield and 1000 grain weight (TGW) data from 432 trials in farmers' fields were evaluated to review long-term yields (1977–2005) and control of eyespot and Leaf Blotch Diseases (LBDs, including Septoria tritici blotch, Stagonospora nodorum blotch and tan spot), powdery mildew, brown rust and yellow rust. Regression analyses revealed that control of LBDs explained 74% of the yield increase achieved by fungicide treatment at GS 45–61, followed by powdery mildew (20%), brown rust (5%) and yellow rust (1%). Yield of both untreated and fungicide-treated plots increased from approx. 6000 to 12 000 kg ha −1 over the period 1983–2005. Single eyespot treatment improved yield by ∼320 kg ha −1 yr −1 during the period 1977–2002, mainly due to occasional years with severe eyespot. Single leaf disease treatment at GS 45–61 increased mean yield by 10.3% or 810 kg ha −1 yr −1 (9.9% or 660 kg ha −1 yr −1 for 1983–1994 and 10.7% or 970 kg ha −1 yr −1 for 1995–2005) due to increased TGW and grain numbers, especially in high-yielding stands. Additional extra early treatment at GS 30–40 against LBDs increased yield by ∼250 kg ha −1 yr −1. Estimated variance in yield and TGW was higher between years than within years, while that in yield increase and plant diseases was lower between years than within. The results confirm potential and limits of fungicides and the need for supervised control strategies including factors affecting disease, yield and interactions.

Arbuscular mycorrhizas contribution to nutrition, productivity, structure and diversity of plant community in mountainous herbaceous grassland of northern Greece

A 2-year study is presented on the impact of arbuscular mycorrhizal fungi (AMF) on plant productivity, uptake of phosphorus (P) and nitrogen (N) and diversity of plant community in a herbaceous, P and N limited grassland consisting exclusively of C3 species and dominated by two plant species less mycotrophic than the subordinate ones. We hypothesized that AMF suppression will increase the abundance of the dominant species with compensatory decreases of subordinate species and decline of the diversity components. Mycorrhizal colonization was suppressed in field plots through application of the fungicide benomyl as a soil drench every 2 weeks during each growing period. Suppression of mycorrhizae lowered P concentrations of the examined species or species groups and decreased the abundance of the P demanding perennial forbs (the non-legume dicots) and most legumes. Concurrently, there were recorded increases in the abundance of the less P demanding grasses, sedges and the legume Doricnium herbaceum. Many rare forb species were not able to establish in the fungicide-treated plots. As a result, in the fungicide-treated plots there was a decline of the diversity components. In spite of the lower efficiency in soil P use and the lower diversity, primary productivity in the fungicide-treated plots did not differ from that of non-treated plots. It is suggested that AMF are important determinants of plant structure and diversity in P limiting soils, and the direction in which fungicide application affects the diversity of plant communities depends on the mycotrophic status of the dominant and subordinate plant species.

Evaluation of fungicide and plant growth regulator tank-mix programmes on dollar spot severity of creeping bentgrass

Dollar spot ( Sclerotinia homoeocarpa F.T. Bennett) is a common and destructive disease of creeping bentgrass ( Agrostis stolonifera L.). In 2003 and 2004, field studies were conducted to evaluate 14- and 21-day sequential fungicide tank-mix programmes alone and in combinations with plant growth regulators (PGRs) paclobutrazol, trinexapac-ethyl, or paclobutrazol plus trinexapac-ethyl on dollar spot of fairway-height creeping bentgrass. All fungicide-treated plots, either fungicides plus PGRs or fungicides alone, exhibited greater dollar spot control versus untreated plots. Overall, fungicide efficacy on dollar spot was comparable on plots treated with fungicide tank-mix programmes alone versus fungicides plus PGRs. Creeping bentgrass quality was consistently better in plots treated with fungicides plus PGRs versus fungicides alone. Dollar spot control and creeping bentgrass quality was improved, however, in plots treated with fungicides plus paclobutrazol versus fungicides alone, fungicides plus trinexapac-ethyl, or fungicides plus paclobutrazol plus trinexapac-ethyl.

Plant pathogens drive density‐dependent seedling mortality in a tropical tree

One explanation for the extraordinary diversity of tropical forest trees is that density-dependent mortality from herbivores or pathogens puts locally rare species at an advantage. Density-dependent mortality of seeds and small seedlings is particularly intense in tropical forests, but its causes remain uncertain. Here, we show experimentally that pathogens from the Oomycota are associated with intense mortality in seedlings of a neotropical tree, Sebastiana longicuspis. Seedlings in untreated plots experienced eight times higher mortality compared with seedlings in plots treated with fungicide. Mortality was strongly density dependent: in fungicide-treated plots survival was unaffected by density, but survival in unsprayed plots was over three times higher at low density. Density-dependent mortality observed in a simultaneous, non-manipulative study was highly transient, suggesting that short-term observational studies may underestimate the intensity and form of pathogen-induced mortality. If such effects are widespread, plant pathogens may play a key role in maintaining and structuring tropical diversity.

Azoxystrobin Rate and Timing Effects on Rice Sheath Blight Incidence and Severity and Rice Grain and Milling Yields.

Lack of sheath blight resistant varieties requires rice (Oryza sativa) farmers to use fungicides to avoid significant reductions in grain yield and milling yield. Studies were conducted to determine the best rate and rice growth stage for a single application of azoxystrobin, a fungicide recently labeled for this use, to effectively control sheath blight. Azoxystrobin was applied foliarly to inoculated field plots in 2000 to 2002 using rates of 0.22, 0.17, and 0.11 kg a.i. ha -1 at 7 days after panicle differentiation (PD+7), boot (B), and heading (H) growth stages and at 0.17 kg a.i. ha-1 at 5, 10, and 15 days after heading. Sheath blight severity (measured on a 0 to 9 scale) and incidence (percent tillers infected) were assessed 1 to 2 weeks before harvest. A fungicide application made between PD+7 and H significantly reduced sheath blight severity, which resulted in significantly higher yield and head rice milling yield compared with inoculated unsprayed plots. There were no significant effects of fungicide rate at the PD+7, B, and H growth stages. Fungicide-treated plots had yield and milling yield similar to the very lightly diseased, uninoculated check plots included for comparison. With fungicide application made postheading, rice had higher disease severity, resulting in lower yields and milling yield compared with rice receiving earlier applications.

Grape downy mildew spread and mite seasonal abundance in vineyards: effects on Tydeus caudatus and its predators

Abstract Tydeus caudatus (Acari: Tydeidae) can prey upon grape eriophyoid mites but little is known about its alternative foods. Observations carried out during 1999–2003 in a commercial vineyard located in northeastern Italy showed that densities of T. caudatus were often correlated to downy mildew spread on the vegetation. Densities of T. caudatus increased in late summer when downy mildew symptoms occurred on a high number of leaves. The predatory mite Paraseiulus talbii (Acari: Phytoseiidae) increased in late season following the increase of tydeids. On several sampling dates, T. caudatus populations were significantly higher on leaves with downy mildew symptoms than on leaves without symptoms. Tydeid densities were often positively correlated to the extent of leaf surface showing symptoms. These relationships were sometimes found regarding P. talbii. In two experimental vineyards colonised by T. caudatus, untreated plots or plots treated with different fungicides to control downy mildew were monitored during 2002. Downy mildew infections rapidly spread in the control plots while they were effectively controlled on fungicide-treated plots. In both vineyards, T. caudatus densities reached significantly higher densities in the control than on fungicide-treated plots. A field study showed that most of these fungicides did not reduce tydeid populations when downy mildew was virtually absent. Isoelectric-focusing electrophoresis (IEF) was used to detect downy mildew in mites. Glucose phosphate isomerase (GPI) was selected from among different enzymes. A high proportion of T. caudatus females, collected from infected leaves, and analysed under IEF showed a GPI isozyme allele corresponding to the downy mildew isozyme in addition to the tydeid intrinsic alleles. This phenomenon was also observed for P. talbii but with a lower incidence. All T. caudatus females confined on symptomatic leaves became positive by IEF after few hours but the same did not occur with P. talbii. A high proportion of P. talbii females became positive after preying on tydeids collected from symptomatic leaves or reared on pollen and then confined on symptomatic leaves.

Management of Phytophthora pod rot disease on cocoa farms in Ghana

From 1991 to 1997, field observations on trials involving the use of metalaxyl and copper-based fungicides were made on farmers’ farms in four Phytophthora megakarya affected cocoa growing regions of Ghana to control Phytophthora pod rot disease. Data on farm management practices, cocoa and shade tree types and densities, plot sizes, yield, land tenure and labour arrangements for farm operations, disease incidence and profitability of disease control were collected. Lower disease incidence and higher yields were recorded on fungicide-treated plots than on the untreated plots. The profitability of fungicide application depended on the level of farm management, nature of land tenure and labour arrangements for farm operations. The challenges involved in conducting trials with active participation by farmers are discussed. The involvement of farmers in the development of disease control programmes is crucial for subsequent adoption of the technology.

Arbuscular mycorrhizas influence plant diversity and community structure in a semiarid herbland

Summary • The contribution of arbuscular mycorrhizas (AM) to plant community structure and diversity is reported here in an annual herbland in southern Australia. • Mycorrhizal colonization was reduced in field plots by applying the fungicide benomyl as a soil drench. The mycorrhiza-responsiveness of plant species was assessed in intact soil cores containing the indigenous AM fungi and in a pot experiment using an isolate of Glomus mosseae. • Glasshouse experiments showed that Medicago minima, Vittadinia gracilis and Velleia arguta were highly mycorrhiza-responsive, Salvia verbenaca became colonized but exhibited no growth response to AM, and Carrichtera annua remained uncolonized. There was no change in plant species richness in mycorrhiza-suppressed field plots but diversity increased owing to an increase in evenness. Treatment had no effect on community productivity and therefore there was no relationship between mycorrhizal effects on diversity and productivity. • Mycorrhizal responsiveness was not a good predictor of species response to suppression of AM in the field. The mycorrhiza-responsive species V. gracilis and V. arguta were not affected by reduced mycorrhizal colonization in fungicide-treated plots, suggesting that competition from the mycorrhiza-responsive dominant M. minima offset the benefits of mycorrhizal association for these species.

Partial resistance to Mycosphaerella pinodes in field pea

The responses of 335 field pea lines originating from more than 30 countries to mycosphaerella blight caused by Mycosphaerella pinodes were evaluated in inoculated field plots in 1994 and 1995. Seven lines (Baccara, Carneval, Danto, Majoret, Miko, PI273605, and Yellowhead) with low area under the disease progress curve (AUDPC) scores were identified. These seven lines, along with the resistant check, Radley, and the susceptible check, 89–195, were further examined for components of partial resistance to the disease in 1996 and 1998. Components evaluated were leaf area with symptoms (LAS), stem area with symptoms (SAS), pod area with symptoms (PAS), and percent seed infection (SI). Differences among pea lines were found in all components. Compared with the susceptible check, Carneval had significantly lower LAS, PAS, and SI in both years and was considered partially resistant to leaf, pod, and seed infection; Danto had significantly lower LAS, SAS, and PAS and was partially resistant to leaf, stem, and pod infection; Yellowhead had significantly lower SAS, PAS, and SI and was partially resistant to stem, pod, and seed infection; Majoret had significantly lower LAS and SAS and was partially resistant to leaf and stem infection; Miko had significantly lower SAS and PAS and was partially resistant to stem and pod infection; PI273605 had significantly lower PAS and SI and was partially resistant to pod and seed infection; and Baccara and Radley had significantly lower SAS and were partially resistant to stem infection only. Plots of Baccara and Yellowhead inoculated with M. pinodes-infected stubble had 10 and 17% reduction in yield, respectively, compared with non-inoculated and fungicide-treated plots and, thus, were considered tolerant to the disease. The remaining lines had yield reductions of 30 to 49%. Key words: Mycosphaerella blight, Mycosphaerella pinodes, Pisum sativum, field pea, partial resistance, disease tolerance

Studies on the effectiveness of metconazole in controlling Fusarium Head Blight caused by Fusarium culmorum in Spring Wheat (Triticum aestivum L.).

Fusarium Head Blight (FHB) is a disease of increasing concern in wheat growing areas of northwestern Europe. The disease causes a reduction in grain yield and quality, and the production of mycotoxins. It is the latter area that elevates this disease to a higher order of importance due to the adverse effects reported on animal and human health. Metconazole was evaluated in trials over a three-year period. The initial trial was a study of the efficacy of metconazole and tebuconazole in the control of FHB in spring wheat. In 1997, ail fungicide treatments had significantly less disease (P<0.01) than the untreated. Metconazole applied 72 hours after inoculation led to a slower rate of disease development than tebuconazole applied after 48 hours. However, there was no significant difference between tebuconazole and metconazole when they were applied at the same timing. When this trial was repeated in 1998, there was a slight, yet statistically significant, difference in favour of the tebuconazole treatment at the first assessment date (21 days post inoculation). Thereafter, fungicide-treated plots had significantly less disease than untreated with no significant differences among individual fungicide treatments. Since strobilurins are now the dominant fungicide chemistry, the effect of metconazole alone and in combination with a strobilurin (azoxystrobin) on FHB was studied in 1998. No significant difference in efficacy was recorded between metconazole and a combination of metconazole and azoxystrobin. Both treatments were significantly better in controlling ear and seed symptoms than azoxystrobin alone and the untreated. Three fungicide application timings were evaluated: a pre, a post and pre+ post inoculation application. Post inoculation (48 hours) treatment with metconazole or metconazole + azoxystrobin was found to be the most effective and no additional control was achieved by splitting the application timing. This trial, with the addition of trifloxystrobin alone and in combination with metconazole was repeated in 1999. Metconazole alone and in mixtures, again, gave significantly better FHB control than all other treatments. Trifloxystrobin was significantly better than azoxystrobin alone in controlling F. culmorum. However, it was not as effective as the metconazole formulations.

A Field Trial of Crown Rust (Puccinia coronata f. sp. avenae) as a Biocontrol Agent of Wild Oats on San Clemente Island

San Clemente Island has been subjected to large-scale disturbance from grazing and subsequent weed infestation, particularly by two wild oat species, Avena fatua and A. barbata. An endemic disease, crown rust (caused by Puccinia coronata f.sp. avenae ), was selected as a potential control agent. A strain with a short latent period was selected from a common island isolate. Three replicate sets of plots were established on the island, each set containing a plot treated with crown rust, a control plot, and a plot treated with a fungicide. Disease was assessed in these plots and outside of the inoculated plots to a distance of 100 m. Reproductive output of wild oats from these plots was measured and compared for differences among treatments. Results indicated that disease was established from artificial inoculations in both years, 2 weeks earlier than the onset of disease from natural infections in 1997, but not in 1998. This result may have been due to unusual weather patterns in 1998 that caused an earlier onset of natural infections and precluded higher success of artificial inoculations early in the season. Analysis of reproductive output of wild oats from these plots showed that biomass and seed number were significantly lower ( P < 0.05) in inoculated plots than in control and fungicide-treated plots. These results indicated that, with augmentation over several years, crown rust may be effective in reducing populations of wild oats on San Clemente Island.

Microbial population dynamics on Golden Delicious apples from bud to harvest and effect of fungicide applications

SummaryThe microbial population dynamics on apples cv. Golden Delicious were analysed every 15 days between bud and harvest in a fully replicated experiment in northern Spain in 1994 and 1995. The total microbial populations varied with developmental stage, and with prevailing climatic conditions. The predominant mycroflora were the filamentous fungi Cladosporium and Alternaria spp. and white and pink yeasts. Other genera isolated included mainly species of Epicoccum, Fusarium and Acremonium. However, the most important post‐harvest pathogens Penicillium expansum and Botrytis cinerea were seldom isolated from ripening apples. Maximum total filamentous fungal populations occurred after fruit set and during early ripening [2 × 104cfu (colony‐forming units) g‐1 approximately] while those of bacteria were maximum at bud stage (3.5 × 105and 3.0 × 104 cfu g‐1 in 1994 and 1995 respectively). White yeasts were more numerous than pink yeasts. Endophytic infection of apple buds by Alternaria spp., responsible for core rot, was found in almost all bud tissue. By contrast, Cladosporium spp. were initially isolated later from 12.5–50% of tissue samples during blooming and fruit set. The impact of a four‐spray fungicide regime during apple development significantly decreased the total filamentous fungal populations in both years, and that of Cladosporium spp. in 1994. However, bacterial populations were often higher on apples from fungicide‐treated plots. Fungicide sprays decreased populations of Cladosporium, Alternaria and white yeasts for a maximum of up to 15–30 days after application. Fungicide application had little effect on endophytic infection of apples by Alternaria spp. between bud and harvest.

Effectiveness of indirect selection for wheat yield in a stress environment

Effectiveness of indirect selection for grain yield was investigated among eight bread wheat lines, selected in a stress environment in the East African Highlands, where they showed a wide response to yellow rust, the biotic factor causing the stress. The lines were intercrossed to give an 8×8F1 diallel, which was grown in this same environment for three consecutive growing seasons during 1994 and 1995. Half of the last trial was sprayed with fungicide. From the estimates of narrow-sense heritabilities for yield and yellow rust severity, and the genetic correlation between them, indirect selection for yield in the unsprayed plots, obtained by choosing the most resistant 5% of this material was, on average, 12% higher than selecting for yield itself. In the fungicide-treated plots, indirect selection was only 73% as efficient as direct selection. The implications of these results for selecting for yield in stress environments are discussed.

Efficacy of Fungicide Treatments for Control of Common Rust and Northern Leaf Spot in Hybrid Corn Seed Production.

Field experiments were conducted during the 1992 and 1993 growing seasons in 13 commercial fields in Iowa to assess the efficacy of foliar applications of fungicides to seed corn for control of common rust, Puccinia sorghi, and northern leaf spot, Bipolaris zeicola. Chlorothalonil, mancozeb, and/or propiconazole sprays were initiated at growth stage (GS) 2.5, GS 4.0, or approximately 2 weeks before detasseling, and applications were repeated up to five times at various growth stages. The majority of fungicide treatments resulted in an increase in salable seed units (1 unit = 80,000 seeds) compared to untreated control treatments. The greatest increase in salable seed occurred in the large and medium seed size units. Although disease severity on the final assessment date in both years and standardized area under the disease progress curve in 1993 were significantly higher (P ≤ 0.05) in untreated control plots than in fungicide-treated plots in all experiments, the best disease control was provided by 5 applications of chlorothalonil or 2 to 4 applications of chlorothalonil, mancozeb, or propiconazole initiated on the first spray date.

Influence of Fungicides on Development of an Entomopathogenic Fungus (Zygomycetes: Neozygitaceae) in the Cotton Aphid (Homoptera: Aphididae)

Fungicides used commercially for control of cotton seedling diseases were evaluated under field conditions during a 3-yr period for their effects on the prevalence of the entomopathogenic fungus Neozygites fresenii (Nowakowski) Batko in the cotton aphid, Aphis gossypii Glover. The susceptibility of N. fresenii to the fungicides varied. Carboxin significantly reduced N. fresenii prevalence during the early season. Furthermore, A. gossypii population levels were significantly greater during the 1993 field test in plots treated with carboxin. In some cases etridiazole and metalaxyl also significantly reduced N. fresenii prevalence during the early season. Conversely, N. fresenii prevalence was generally equivalent to or greater in the fungicide-treated plots than in the control plots during the late season. Although numerous biotic and abiotic factors (that is, environmental or artificial or both) also influence fungal prevalence in field situations, this appears to be the 1st demonstration of reduced entomopathogenic fungal prevalence caused by the application of a granular fungicide at planting. Biological and cultural implications are discussed.