Leaf Shredding Research Articles

Salamander loss alters montane stream ecosystem functioning and structure through top‐down effects

AbstractAmphibians are among the most endangered taxa worldwide, but little is known about how their disappearance can alter the functioning and structure of freshwater ecosystems, where they live as larval stages. This is particularly true for urodeles, which often are key predators in these ecosystems. The fire salamander (Salamandra salamandra) is a common predator in European fresh waters, but the species is declining due to habitat loss and the infection by fungal pathogens. We studied the consequences of fire salamander loss from three montane streams, by comparing two key ecosystem processes (periphyton accrual and leaf litter decomposition) and the structure of three communities (periphytic algae, aquatic hyphomycetes and invertebrates) using stream enclosures with and without salamander larvae. Salamander loss did not cause changes in invertebrate abundance or community structure, except for one stream where abundance increased in the absence of salamander larvae. However, salamander loss led to lower periphyton accrual, changes in algal community structure and slower leaf litter decomposition, with no associated changes in fungal communities or microbial decomposition. The changes observed may have been caused by release of salamander predatory pressure on invertebrates, which could have promoted their grazing on periphyton, in contrast to their preference for leaf shredding in the presence of salamander. Our study demonstrates an important role of salamander larvae in montane streams through top‐down control of lower trophic levels and thus in regulating key stream ecosystem processes. Our results highlight the need for improving protection measures for amphibians to prevent these alterations on ecosystem structure and function.

Leaf shredding as an alternative strategy for managing apple scab resistance to demethylation inhibitor fungicides

Within integrated apple scab control there is a strong focus on reduction of Venturia inaequalis primary inoculum. The hypothesis that leaf shredding as an orchard sanitation practice would reduce the effective population size of the fungus (resulting in lower genetic variation due to reduction in sexual offspring) was tested. Assuming the allele causing fungicide resistance is already present in the population, it will be widely distributed at the end of the season, since selection occurs when the demethylation inhibitor (DMI) fungicide was applied. For short-term disease management a reduction of inoculum size, (i.e. potential ascospore dose) is most important. In the long-term resistant isolates/genotypes would be less likely to survive the winter and/or to infect in the spring, if that inoculum (i.e. in fallen leaves) has been removed. To sustain the use of highly effective synthetic fungicides, such as the DMIs, fungicide resistance management practices have to be evaluated. Fungicide resistance, which negatively affects pathogen fitness, is hypothetically reversible, if the selection pressure by the fungicide is removed. This study quantified the effect of leaf shredding on changes in the pathogen’s flusilazole sensitivity and population genetic structure using SSR markers. Venturia inaequalis populations in orchard trials, where sanitation practices had been applied, were tested for flusilazole sensitivity in planta and in vitro. Significant shifts towards flusilazole resistance were identified in orchards with a history of DMI application without sanitation treatment, with a mean sensitivity of EC50= 0.208 ug/ml (n=49) compared to an unexposed V. inaequalis population (EC50= 0.104 ug/ml, n=55). However, the isolates from the same sanitation trial orchards, from leaf shredding treatment in combination with a fungicide spray programme, had a mean EC50 of 0.110 ug/ml (n=41), similar to an unexposed V. inaequalis population. Furthermore, V. inaequalis offspring after sanitation treatment, showed shifts in microsatellite allele frequency distribution patterns used as an indicator of sexual reproduction. This study concludes that sanitation treatments, i.e. leaf shredding, impact on fungicide sensitivity and therefore effectively contributes to fungicide resistance management.

Effect of Urea Fertilizer Dosage and Under-Cob Leaf Removal on Growth and Production of Three Maize Varieties

The assimilation products produced by the leaves will be distributed to all parts of the plant that need it. The presence of leaves can help smooth assimilation, but can also be a user of assimilate products. The removal of leaves under the cob is done to streamline the process of photosynthesis that occurs in old leaves that cause moisture, it is also intended to suppress the occurrence of internal competition in assimilation, while the need to be given the right fertilizer or nutrition for the growth and production of corn which is needed, namely Nitrogen in Urea Fertilizer. This study aims to analyze the growth and production of three varieties of corn with the application of different doses of urea fertilizer and leaf shredding under the cob. The research was conducted in the form of a multifactor experiment arranged based on a Split Split Plots Design (SSP) in a Randomized Group Design (RAK) pattern consisting of the Main Plots Factor (MP), namely Maize Variety Type consisting of Agricon, BISI-10,and C7 Varieties. Sub Plots (SP) are doses of urea fertilizer of 250 kg.ha-1 and 350 kg.ha-1 respectively, and Sub Sub Plots (SSP) are the method of leaf trimming under the cob consisting of No trimming. Composting at anthesis, and Composting 1 week after anthesis. The results of the analysis showed that the Bisi-10 variety gave a better response to the dose of urea fertilization at the vegetative growth stage of corn plants and the Agricon variety gave a better response to urea fertilization and pumping time at the generative growth stage, the dose of urea fertilizer 350 kg ha-1 gave a better response to the growth and development of corn plants and corn plant production and the time of pumping the leaves under the cob at the time of pollination was more effective in increasing the quality and quantity of corn plant production.

Differential resource consumption in leaf litter mixtures by native and non-native amphipods

Leaf litter processing is an essential ecosystem function in freshwater systems, since much of the carbon and nutrients moving through freshwater food webs come from the surrounding terrestrial ecosystems. Thus, it is important to understand how the species performing this function differ, especially because many native species are being replaced by non-native species in aquatic ecosystems. We used a field experiment to examine leaf consumption rates of two common shredding macroinvertebrates (the native Gammarus fossarum and the non-native Gammarus roeselii). Leaves from three species, varying in resource quality, were added both in leaf monocultures and as a three-species mixture. Biomass-adjusted daily consumption rates were similar between the two amphipod species, and each consumed nitrogen-rich alder leaves faster than oak or beech leaves. However, because adult G. roeselii are approximately twice the size of G. fossarum, this led to systematic, though nonsignificant, differences in consumption rates at the per-capita or population level. Furthermore, we found nuanced effects of decomposer identity on leaf decomposition in mixtures. Only G. roeselii showed increased consumption of the preferred resource (alder) in the mixture, while G. fossarum consumed all leaves at the same proportional rates as in monocultures. This is an important distinction, as most measures of macroinvertebrate leaf shredding are made in the laboratory with only a single leaf resource available. Our results, based on a field experiment which could control the presence of dominant macroinvertebrates while still providing natural, biologically realistic context, suggest that even functionally similar species may subtly shift ecosystem processes.

Optimization of a Whole-Stalk Operating System after Sugarcane Base Cutting

Abstract. Leaf stripping and top breaking are the key procedures in whole-stalk sugarcane harvesting. In this study, a whole-stalk operating system after sugarcane base cutting was developed and tested to improve the leafy trash cleaning performance and adaptability of a whole-stalk harvester. A leaf shredding procedure was proposed before leaf stripping, and an anti-float top breaking device was developed for breaking cane tops. The rotational speeds of the leaf shredding (RL), leaf stripping (RS), and top breaking (RT) rollers were regarded as experimental factors, and the leafy trash content (including leaves, leaf sheaths, and tops in this study), top breaking rate, abrasion rate, and non-fracture rate of the middle or bottom parts were selected as indices. Central composite design combined with response surface methodology and comprehensive evaluation method were employed to conduct experiments and explore the interaction effects of factors on indices. The optimal operating parameters were nonlinearly optimized and determined to be RL of 512.9 rpm, RS of 418.8 rpm, and RT of 307.0 rpm. Under these conditions, the predicted values of leafy trash content, top breaking rate, abrasion rate, and non-fracture rate were 4.98%, 88.39%, 5.19%, and 96.21%, respectively. Verification experiments indicated that the observed values were in agreement with the predicted values. Results suggested that the leaf shredding device and anti-float top breaking device developed in this study were effective for improving the leafy trash cleaning performance and adaptability of the whole-stalk operating system, and there was a good potential for performance improvement of whole-stalk sugarcane harvesters. Keywords: Central composite design, Leaf shredding, Leaf stripping, Response surface methodology, Top breaking, Whole-stalk harvester.

The Effect of Leaf Shredding on Apple Scab in South African Orchards.

The South African apple industry currently relies entirely on chemical fungicides to control apple scab (Venturia inaequalis). In this study, the effectiveness of sanitation strategies in reducing scab incidence and severity in South African orchards was evaluated. Over three seasons, leaf shredding with no fungicide sprays was tested against a nonsprayed, nonshredded negative control, a positive control that followed a commercial fungicide program, and a combined treatment of a commercial fungicide program with leaf shredding. Two treatment replicates were applied in a randomized block design in each of two orchards. Scab incidence and severity on fruit and leaves were assessed weekly from green-tip until fruit-set in the following spring. Pooled data from the 3 years revealed that fruit scab incidence and severity and leaf scab severity (51, 55, and 39%, respectively, P < 0.05) and leaf scab incidence (33%, P < 0.1) were significantly lower in the leaf-shredding treatment than in the negative control. This is the first study to evaluate the effect of leaf shredding in reducing scab in South African orchards. Results indicate that this treatment is highly effective and should be integrated into scab management strategies in future, but should be customized to suit South African orchard conditions.

The impact of the invasive amphipod Dikerogammarus haemobaphes on leaf litter processing in UK rivers

The impact of invasive gammaridean species on native biodiversity is well documented, but the potential for them to disrupt ecosystem functioning is less well understood. Native freshwater amphipods are considered to be archetypal leaf shredders and are considered key to leaf litter processing within rivers. It is possible that invasions may interfere with key ecosystem processes such as leaf litter breakdown, due to behavioural traits displayed by the invasive species. In two laboratory experiments, we compared the leaf shredding efficiency of the native Gammarus pulex and Dikerogammarus haemobaphes, a recently established Ponto–Caspian invader in the UK. We hypothesised that in isolation G. pulex would have a greater shredding efficiency than D. haemobaphes and that, in the presence of the invasive, leaf shredding and survival of G. pulex would be reduced. The results supported our hypothesis that, in isolated conditions, G. pulex consumed significantly more leaves than D. haemobaphes. Under mixed treatments, G. pulex leaf consumption and survival, although not statistically significant, appeared to be reduced. The implications of our findings suggest that the potential displacement of G. pulex from its native range, by D. haemobaphes, could lead to a decline in leaf litter processing and recycling in rivers within the UK.

Direct and indirect toxicity of the fungicide pyraclostrobin to Hyalella azteca and effects on leaf processing under realistic daily temperature regimes

Fungicides in aquatic environments can impact non-target bacterial and fungal communities and the invertebrate detritivores responsible for the decomposition of allochthonous organic matter. Additionally, in some aquatic systems daily water temperature fluctuations may influence these processes and alter contaminant toxicity, but such temperature fluctuations are rarely examined in conjunction with contaminants. In this study, the shredding amphipod Hyalella azteca was exposed to the fungicide pyraclostrobin in three experiments. Endpoints included mortality, organism growth, and leaf processing. One experiment was conducted at a constant temperature (23 °C), a fluctuating temperature regime (18–25 °C) based on field-collected data from the S. Llano River, Texas, or an adjusted fluctuating temperature regime (20–26 °C) based on possible climate change predictions. Pyraclostrobin significantly reduced leaf shredding and increased H. azteca mortality at concentrations of 40 μg/L or greater at a constant 23 °C and decreased leaf shredding at concentrations of 15 μg/L or greater in the fluctuating temperatures. There was a significant interaction between temperature treatment and pyraclostrobin concentration on H. azteca mortality, body length, and dry mass under direct aqueous exposure conditions. In an indirect exposure scenario in which only leaf material was exposed to pyraclostrobin, H. azteca did not preferentially feed on or avoid treated leaf disks compared to controls. This study describes the influence of realistic temperature variation on fungicide toxicity to shredding invertebrates, which is important for understanding how future alterations in daily temperature regimes due to climate change may influence the assessment of ecological risk of contaminants in aquatic ecosystems.

Pronounced species turnover, but no functional equivalence in leaf consumption of invasive amphipods in the river Rhine

A growing body of literature focuses on the adverse effects of biological invasions, e.g., on the decline of indigenous biodiversity, while studies on the consequences of invasions on components of ecosystem functioning are comparatively rare. Owing to their leaf shredding activity, amphipods play a fundamental role in determining energy flow dynamics in Central European freshwater ecosystems, but whether the dramatic change in species composition after the invasion of Ponto–Caspian taxa affects this process has not been addressed in a comprehensive study. In a laboratory experiment we determined consumption rates of three leaf types (Alnus glutinosa, Betula pendula, Quercus robur) from common riparian arboreal vegetation in the Rhine drainage—one of the most heavily invaded river systems worldwide—by the most common native (Gammarus fossarum, G. pulex, G. roeselii) and invasive amphipods (Dikerogammarus villosus, Echinogammarus ischnus). Leaf-shredding activity was significantly lower in invasive than in native amphipods across leaf types, and a subsequent analysis ruled out an effect of different metabolic rates as an explanation. Another experiment was motivated by the observation that native amphipods are nowadays restricted to smaller tributaries to the Rhine, while invasive taxa are dominant in the main channel. As leaf litter shredding may be more important in headwaters than in lower parts of streams, we sought for a signature of within-species variation in the feeding ecology of amphipods and thus compared two different populations of G. pulex, but found very similar leaf consumption rates in upstream and downstream populations, suggesting that food preferences in amphipods could be species-specific with little potential for microevolution or environmentally induced plasticity. In conclusion, the rapid replacement of native amphipod species in the Rhine drainage likely affects vital ecosystem services, with the potential to change the aquatic food web (e.g., through reduced shredding activity and hence, reduced resource availability for particle-feeding detritivores), unless other taxonomic groups compensate for those functional alterations.

Development of a Semi-Automated Tobacco Stripping Machine Utilizing String Trimmers

<abstract><title><italic>Abstract.</italic></title> Conventional stripping of burley tobacco is labor intensive and typically requires 50 to 75 worker hours per acre (wkr•h/acre). The goal of the project was to reduce labor by optimizing leaf removal by string trimmer heads using combinations of strings lengths and motor speeds. In tests conducted on a single grade, all leaves outside the grade were removed by hand. Plants were run through the machine for string lengths of 13, 18, and 23 cm (5, 7, and 9 in.) and associated motor speeds which were monitored and recorded. Stripping efficiencies were calculated for each plant and collectively for each set of four plants. The machine was then tested for three grade stripping efficiency. Particle size analysis tests were run to determine potential losses due to leaf shredding. Efficiencies for single grade testing ranged from 93% to 96% for optimal string length and speed combinations. Stripping three grades by machine resulted in an average of 97% efficiency. Potential losses due to shredding accounted for 5.6% of the total weight mechanically removed. It is believed that this stripping concept, implemented on a full scale four grade basis, could result in savings of at least 45 wkr•h/ha (18 wkr•h/acre).

Reducing Primary Inoculum of Apple Scab Using Foliar Application of Urea in Autumn

The effect of applications of urea 5% after harvest but before leaf-fall, as foliarapplication, in order to restrict perithecial production by Venturia inaequalis in a commercial superintensive apple orchard situated near Cluj-Napoca, Romania was studied. The urea works in two ways:directly by inhibiting the development of ascospores, and by stimulating the development ofantagonistic microorganisms from against V. inaequalis. There are several possibilities to reduce theamount of overwintering apple scab, such as removing fallen leaves by raking, vacuuming, or usingleaf blowers, chopping or shredding the leaf litter in the spring. Primary infection of scab conidia isproduced by infected leaf litter on the orchard floor. Any modalities that reduce the amount of leaflitter will help to reduce the amount of inoculum for primary infection with scab. Urea-treated leavesdecomposed rapidly infected leaf litter, thus destroying the overwintering substrate for the V.inaequalis fungus. The results have shown large reductions in spore production, often as high as 70 to80%, following application of 5% urea. Spraying the surface of the leaves on the ground with urea 5%reduced primary infection by about 60%. In conclusion, urea applications proved to be effective toreduce apple scab inoculum. Combining the urea with other modalities of leaf shredding provides thegreatest reductions in ascospores.

Fungal composition on leaves explains pollutant-mediated indirect effects on amphipod feeding

The energy stored in coarse particulate organic matter, e.g. leaf litter, is released to aquatic ecosystems by breakdown processes involving microorganisms and leaf shredding invertebrates. The palatability of leaves and thus the feeding of shredders on leaf material are highly influenced by microorganisms. However, implications in the colonization of leaves by microorganisms (=conditioning) caused by chemical stressors are rarely studied. Our laboratory experiments, therefore, investigated for the first time effects of a fungicide on the conditioning process of leaf material by means of food-choice experiments using Gammarus fossarum (Crustacea: Amphipoda). Additionally, microbial analyses were conducted to facilitate the mechanistic understanding of the observed behavior. Gammarids significantly preferred control leaf discs over those conditioned in presence of the fungicide tebuconazole at concentrations of 50 and 500 μg/L. Besides the decrease of fungal biomass with increasing fungicide concentration, also the leaf associated fungal community composition showed that species preferred by gammarids, such as Alatospora acumunata, Clavariopsis aquatica, or Flagellospora curvula, were more frequent in the control. Tetracladium marchalianum, however, which is rejected by gammarids, was abundant in all treatments suggesting an increasing importance of this species for the lower leaf palatability – as other more palatable fungal species were almost absent – in the fungicide treatments. Hence, the food-choice behavior of G. fossarum seems to be a suitable indicator for alterations in leaf associated microbial communities, especially fungal species composition, caused by chemical stressors. Finally, this or similar test systems may be a reasonable supplement to the environmental risk assessment of chemicals in order to achieve its protection goals, as on the one hand, indirect effects may occur far below concentrations known to affect gammarids directly, and on the other hand, the observed shifts in leaf associated microbial communities may have perpetuating implications in leaf shredding invertebrates.

Infection Potential of Pleospora allii and Evaluation of Methods for Reduction of the Overwintering Inoculum of Brown Spot of Pear.

The capacity for germination and pathogenicity to pear leaves of ascospores of Pleospora allii, the teleomorph of Stemphylium vesicarium, causal agent of brown spot of pear, were studied in vitro. Most ascospores germinated within 1 h at temperatures between 15 and 20°C, and the optimum temperature for germination was 18.9°C. Infections developed on wounded and non-wounded detached pear leaves, but were more frequent on wounded leaves. The minimum infective dose was one ascospore per wound. Biological, chemical, and mechanical methods for decreasing overwintering inoculum of P. allii were evaluated. Ascospores were discharged from March to May, depending on the orchard and year. Leaf shredding or removal were the most effective methods of reducing overwintering inoculum. Biological control methods based on application of Thichodermasp. formulations were partially effective. Chemical methods based on copper and urea treatments were ineffective.

Effects of integrated control measures on earthworms, leaf litter and Venturia inaequalis infection in two European apple orchards

Two-, three-year studies were conducted to determine the effect of combining chemical (fungicide and urea) and non-chemical (leaf shredding) sanitation treatments on earthworm populations, leaf litter density (LLD), and leaf and fruit infections caused by Venturia inaequalis in Dutch and Hungarian integrated apple orchards from 2000 to 2002 and 2001 to 2003, respectively. The combined sanitation treatment included urea and captan sprays applied to the tree and to the orchard floor and the shredding of fallen leaves in the previous autumn and winter. The earthworm numbers were significantly higher in the sanitation treatment compared to the treatment without sanitation in 2001 and 2002 (P < 0.05) at Randwijk (The Netherlands) and in 2002 and 2003 (P < 0.05) at Derecske (Hungary). LLD decreased continuously from leaf fall in late autumn to mid-May the following spring at Randwijk, which was explained well by a linear function fitted to the LLD data, with R2 values ranging from 0.82 to 0.99. Model coefficients for the sanitation plots were significantly different (P < 0.05) from those of the non-sanitized plots in 2001 and 2002. At Derecske, LLD started to clearly decrease in both treatments from February 2001 and 2002 and from March 2003 to late spring, and this was explained well by an exponential function fitted to the LLD data, with R2 values ranging from 0.80 to 0.97. Model coefficients for the sanitation plots were significantly different (P < 0.05) from those of the non-sanitized plots only in 2002. In most years and at both sites, the sanitation treatment resulted in significantly lower (P < 0.05) scab incidence on spur-leaf clusters compared to the treatment without sanitation. However, the sanitation effect on scab levels on older leaves and fruits was low and varied by year and location. The results are discussed in connection with the environmental conditions in the two countries.

Texas Has a New Pathotype of Peronosclerospora sorghi, the Cause of Sorghum Downy Mildew.

Three pathotypes of Peronosclerospora sorghi were known to occur in Texas as of 1980, with pathotype 3 (P3) predominant on sorghum (Sorghum bicolor) grown in the Upper Coast area. Following the use of hybrids resistant to P3, combined with metalaxyl or mefenoxam seed treatment, sorghum downy mildew (SDM) became a minor disease in Texas until the occurrence of a widespread outbreak caused by a P3 strain resistant to metalaxyl and mefenoxam in Wharton County in 2001 (2). During July 2004, <1% of plants in a commercial field in Wharton County planted to two Pioneer Brand P3-resistant hybrids had white stripes on the leaves and leaf shredding typical of systemic SDM. To obtain inoculum for pathogenicity studies, several infected plants were removed from the field and transplanted to pots for growth in a greenhouse. Systemically infected leaves suitable for inoculum production subsequently developed from tillers. Conidia were collected from leaves using a tiered temperature system (1). One-week-old seedlings of 10 sorghum lines used as pathotype differentials for Texas were sprayed until runoff with a conidial suspension (8 × 104 per ml) and incubated for 24 h at 20°C and 100% relative humidity. Seedlings were grown for 6 days in the greenhouse and then incubated overnight at 20°C and 100% relative humidity to promote sporulation of lesions, while systemic symptoms were evaluated after an additional 2 weeks in the greenhouse. There were 10 seeds planted per replicate and four replicates per line. The experiment was repeated once. Sporulation occurred on 54 and 64% of plants, and systemic symptoms on 53 and 82% of plants of P3-resistant line SC155 in two experiments, respectively. There were no local lesions or systemic symptoms on SC155 plants inoculated with several P3 isolates. Lines SC414-12E, QL3-India, 82BDM499, and 85EON495, which are resistant to P3, were also resistant to this isolate (i.e., no local lesions or systemic symptoms). Lines RTx2536, RTx430, CS3541, RTx7078, and SC170-6-17, which are susceptible to P3, were also susceptible to this isolate (54 to 100% incidence of systemic symptoms). An experiment (repeated once) that compared the reaction of this new pathotype on metalaxyl-treated and nontreated seed of a P3-resistant hybrid and a P3-susceptible hybrid, with reactions of metalaxyl-resistant P3 isolate and a metalaxyl-sensitive P3 isolate, showed that this pathotype, in addition to overcoming the genetic resistance, was also fungicide resistant. The pathogenicity of this new pathotype to other commercial P3-resistant hybrids is not yet known. There was no yield loss associated with this outbreak. However, the presence of a new pathotype, in combination with fungicide resistance, could lead to further outbreaks of SDM in the Upper Coast of Texas with the potential for yield loss.

A review of non-fungicidal approaches for the control of apple scab

Apple scab is the single most important disease of apple in Canada and the most costly to control. Failure to control apple scab results in severe yield losses and a reduction in market value of harvested fruits. Currently, the strategy to control apple scab relies on multiple applications of fungicides. These sprays are a significant cost to growers and the indirect environmental impact may be substantial. Reliance on fungicides can be reduced by the integration of non-fungicidal control measures that include genetic, physical, and biological approaches. Recommendations for chemical control of apple scab are often based on the highly susceptible cultivar McIntosh. This was justified because up to 60 to 70% of orchards in northeastern North America were planted with this cultivar. Today, the situation is changing as more and more growers are planting new cultivars, some of which are less susceptible to apple scab. A change in the recommendations to account for cultivar susceptibility could result in a reduction in the number of fungicide applications required. In addition, there has been long-standing research on resistant cultivars, but none of the scab-resistant cultivars have been widely accepted. However, with new molecular techniques to identify and locate the resistance genes, there is potential for progress on this front. Furthermore, fungicide applications for primary infections can be delayed if the number of ascospores is reduced after fall treatments that include leaf shredding, the application of biological control agents, or urea. These methods are preventive and can be integrated into existing management programs. However, the integration of all or some of these methods is more complex than simply the use of fungicides. Nevertheless, integrated management of apple scab may prove more sustainable on a long-term basis, mainly because it does not depend on the use of a single method. Hence the risk of the development of fungicide resistance in the pathogen population is reduced.

Apple leaf shredding as a non-chemical tool to manage apple scab and spotted tentiform leafminer

The effect of leaf-shredding on Venturia inaequalis , and Phyllonorycter blancardella and its parasitoids was studied under orchard conditions, respectively, during four and three overwintering periods. Naturally scab-infected leaves were collected in the fall. Five treatments were compared in 1995 and 1996, i.e. shredded leaves, urea (5%), Microsphaeropsis ochracea , Athelia bombacina and untreated leaves (control). In 1997 and 1998, two more treatments were added, i.e. shredded leaves treated with 5% urea and shredded leaves treated with M. ochracea . The ascospore production from a sample corresponding to the average area of nine leaves was evaluated for each spore ejection period from late April to early July. All treatments applied in fall 1995 and 1996 significantly reduced ascospore production in 1996 and 1997, and there was no significant difference among treatments. The most efficient treatment was urea followed by leaf shredding, M. ochracea , and A. bombacina , with reduction in ascospore production of 92.1, 85.2, 84.8, and 80.6%, respectively. In 1998 and 1999, highest reduction in ascospore production was obtained for combined treatments, i.e. shredding and M. ochracea or shredding and urea, with 93.9 and 90.5% reduction in ascospore production, respectively. Apple leaf-shredding in the fall of 1994, 1995 and 1996 significantly reduced emergence of both adult P. blancardella and adult parasitoids associated with the leafminer . Leaf-shredding is a sanitation practice that should be systematically done as a long-term tactic as part of a sustainable integrated pest management program in apple orchards.

Reaction of maize, sorghum and Johnson grass to Peronosclerospora sorghi

Development of sorghum downy mildew, incited by Peronoscleospora sorghi (Weston and Uppal) C.G. Shaw, on maize, sorghum and Johnson grass was investigated at two locations in Uganda during three seasons (1994 and 1995). More sorghum downy mildew developed on the Johnson grass and sorghum than on the maize at all locations and in all seasons. No significant differences were observed in sporulation of P. soghi on the three hosts. Leaf shredding occurred on the three hosts but was the least on maize. Cross-inoculation with both conidia and oospores was achieved on the three hosts. Since the fungal population formed oospores and sporulated readily on the three hosts, which is typical of the sorghum strain, the disease in Uganda is attributed to the sorghum strain.

Effect of wind on the dispersal of oospores of Peronosclerospora sorghi from sorghum

The effect of wind on the dispersal of oospores of Peronosclerospora sorghi, cause of sorghum downy mildew (SDM) is described. The oospores are produced within the leaves of aging, systemically infected sorghum plants. These leaves typically undergo shredding, releasing oospores into the air. Oospores are produced in large numbers (6.1 × 103 cm−2 of systemically infected leaf) and an estimate of the settling velocity of single oospores (0.0437 m s−1) of P. sorghi indicated their suitability for wind dispersal. In wind tunnel studies wind speeds as low as 2 m s−1 dispersed up to 665 oospores per m3 air from a group of leaves previously exposed to wind and displaying symptoms of leaf shredding. The number of oospores dispersed increased exponentially with increasing wind speed. At 6 m s−1, up to 12 890 oospores per m3 air were dispersed. Gusts increased oospore dispersal. A constant wind speed of 3 m s−1 dispersed a mean of 416 oospores per m3. When gusts were applied the mean was 15 592 oospores per m3. In field experiments in Zimbabwe, oospores were sampled downwind from infected plants in the field and at a height of 3.8 m above ground level immediately downwind of an infected crop. These data indicate that wind could play a major role in the dispersal of oospores from infected plants in areas where SDM infects sorghum, perhaps dispersing oospores over long distances.

Red Maple (Acer rubrum) Leaf Decomposition in an Acid-Impacted Stream in Northcentral Pennsylvania

ABSTRACT Decomposition rates of red maple (Acer rubrum) leaves in a variable pH environment were examined in Johnson Creek, a northcentral Pennsylvania acid-mine impacted stream. Two types of red maple leaf packs were placed at four locations along an increasing pH gradient ranging from 5.5 to 7.0. Fine-meshed nylon stocking packs were used to measure chemical and microbial decomposition rates and to exclude stream invertebrates; and, coarse-meshed nylon packs were used to allow for colonization of leaf shredding invertebrates. Differences in decomposition rates for both types of leaf packs were correlated less with increasing pH and more with condition of substrate, stream discharge, and invertebrate community structure. Leaf decomposition rates were significantly slower for the fine-meshed packs than for coarse-meshed packs at all sampling sites. This comparison suggests an important role for both stream discharge and shredder occurrence and density in determining faster decomposition rates in Johnson C...