Rain Splash Research Articles

Cultivar architecture modulates spore dispersal by rain splash: A new perspective to reduce disease progression in cultivar mixtures.

Cultivar mixtures can be used to improve the sustainability of disease management within farming systems by growing cultivars that differ in their disease resistance level in the same field. The impact of canopy aerial architecture on rain-splash dispersal could amplify disease reduction within mixtures. We designed a controlled conditions experiment to study single splash-dispersal events and their consequences for disease. We quantified this impact through the spore interception capacities of the component cultivars of a mixture. Two wheat cultivars, differing in their aerial architecture (mainly leaf area density) and resistance to Septoria tritici blotch, were used to constitute pure stands and mixtures with 75% of resistant plants that accounted for 80% of the canopy leaf area. Canopies composed of 3 rows of plants were exposed to standardized spore fluxes produced by splashing calibrated rain drops on a linear source of inoculum. Disease propagation was measured through spore fluxes and several disease indicators. Leaf susceptibility was higher for upper than for lower leaves. Dense canopies intercepted more spores and mainly limited horizontal spore transfer to the first two rows. The presence of the resistant and dense cultivar made the mixed canopy denser than the susceptible pure stand. No disease symptoms were observed on susceptible plants of the second and third rows in the cultivar mixture, suggesting that the number of spores intercepted by these plants was too low to cause disease symptoms. Both lesion area and disease conditional severity were significantly reduced on susceptible plants within mixtures on the first row beside the inoculum source. Those reductions on one single-splash dispersal event, should be amplified after several cycle over the full epidemic season. Control of splash-dispersed diseases within mixtures could therefore be improved by a careful choice of cultivars taking into consideration both resistance and architecture.

The influence of climate and microclimate (aspect) on soil creep efficiency: Cinder cone morphology and evolution along the eastern Mediterranean Golan Heights

AbstractAlthough hillslope evolution has been subject to much investigation for more than a century, the effect of climate on the morphology of soil‐mantled hillslopes remains poorly constrained. In this study, we perform numerical simulations of volcanic cinder cones in the Golan Heights (eastern Mediterranean) to estimate soil transport efficiency across a significant north–south gradient in mean annual precipitation (1100 to 500 mm). We use the initial cinder cone morphology (constrained by stratigraphy), the modern hillslope form (surveyed with sub‐meter accuracy) and the eruption age (based on 40Ar–39Ar chronology) to predict the best‐fit value of the soil transport coefficient (‘diffusivity’) based on a nonlinear transport model. Our results indicate that the best‐fit diffusivity (K) varies from 1 to 6 m2 ka−1 among the five cinder cones in our field area. Diffusivity (K) values vary systematically with precipitation and hillslope aspect; specifically, K is higher on south‐facing (drier) hillslopes and decreases with mean annual precipitation. We interpret this climate dependency to reflect vegetation‐driven variations in apparent soil cohesion, which increases with root network density, and attenuation of rain splash and overland flow erosion, which increases with vegetative ground cover. To assess how vegetative root mass and ground cover vary with precipitation and aspect, we quantified the spatial distribution of NDVI (normalized difference vegetation index) from ASTER satellite images and observed spatial variations that correlate with our calibrated values of K. Analysis of previously studied cinder cones in the USA can be used to extend our framework to arid domains. This endeavor suggests a humped relationship between K and precipitation with maximum diffusivity at mean annual precipitation of 400–600 mm. Copyright © 2017 John Wiley & Sons, Ltd.

The changing nature of rainfall during the early history of Mars

Several explanations have been proposed for the temporal differences in geologic processes associated with the modification of martian impact craters, which occurred throughout the Noachian, and the formation of valley networks, which occurred during the Noachian/Hesperian transition. Here we show that it could be a result of the changing nature of rainfall as the primordial atmospheric pressure on Mars waned through time. We calculate the terminal velocity and resulting kinetic energy from raindrops > 0.5mm in diameter that would impact the surface of Mars in a CO2-rich atmosphere ranging in pressure from 0.5 to 10bars. Our analyses indicate that the primordial atmosphere of Mars could not have exceeded ∼4.0bars as raindrop sizes would have been limited to < 3mm and surface erosion from rain splash and subsequent crater modification would not have occurred. At pressures between ∼3.0 and 4.0 bars, sediment transport from rain splash could occur, but surface runoff would have been limited, which could explain the modification of impact craters. Once atmospheric pressures waned to ∼1.5bars, rainfall intensity could begin to exceed the infiltration capacity of most soils, which would be necessary to initiate martian valley network formation. Due to the lower gravity, a storm on Mars that occurred in a 1bar atmosphere could generate raindrops with a maximum diameter of ∼7.3mm compared to 6.5mm on the Earth. However, rainfall from such a storm would be only be ∼70% as intense on Mars, primarily due to the lower martian gravity and resulting lower terminal velocities of the rain drops.

Design and Implementation of an Intelligent Windowsill System Using Smart Handheld Device and Fuzzy Microcontroller.

With the advance of science and technology, people have a desire for convenient and comfortable living. Creating comfortable and healthy indoor environments is a major consideration for designing smart homes. As handheld devices become increasingly powerful and ubiquitous, this paper proposes an innovative use of smart handheld devices (SHD), using MIT App Inventor and fuzzy control, to perform the real-time monitoring and smart control of the designed intelligent windowsill system (IWS) in a smart home. A compact weather station that consists of environment sensors was constructed in the IWS for measuring of indoor illuminance, temperature-humidity, carbon dioxide (CO2) concentration and outdoor rain and wind direction. According to the measured environment information, the proposed system can automatically send a command to a fuzzy microcontroller performed by Arduino UNO to fully or partly open the electric curtain and electric window for adapting to climate changes in the indoor and outdoor environment. Moreover, the IWS can automatically close windows for rain splashing on the window. The presented novel control method for the windowsill not only expands the SHD applications, but greatly enhances convenience to users. To validate the feasibility and effectiveness of the IWS, a laboratory prototype was built and confirmed experimentally.

Interrill soil erosion processes on steep slopes

To date interrill erosion processes are not fully understood under different rainfall and soil conditions. The objectives are to 1) identify the interrill erosion regime and limiting process under the study condition, 2) characterize the interactive effects of rainfall intensity and flow depth on sediment transport competency and mode, and 3) develop a lumped interrill erosion model. A loess loam soil with 39% sand and 45% silt was packed to flumes and exposed to simulated rainfall. A complete factorial design with three factors was used, which included rainfall intensity (48, 62, 102, 149, and 170mmh−1), slope gradient (17.6, 26.8, 36.4, 46.6, and 57.7%), and slope length (0.4, 0.8, 1.2, 1.6, and 2m). Rain splash, sediment discharge in runoff, and flow velocity were measured. Results showed that rainfall intensity played a dual role not only in detaching soil materials but also in enhancing sediment transport. Sediment transport was the process limiting interrill erosion rate under the study condition. Two major sediment transport modes were identified: rainfall-driven rolling/creeping and flow-driven rolling/sliding. The relative importance of each mode was largely determined by flow depth. The competence of the flow in transporting sediment decreased downslope as flow depth increased due to increased dissipation of raindrop energy. The optimal mean flow depth for the maximal interrill erosion rates was <0.1mm, which is much shallower than the widely reported 2mm. Slope length was negatively related to interrill erosion rate. The negative correlation seemed stronger for heavier rains, indicating the cushioning effects of flow depth. Lumped interrill erosion models, developed from short slopes, are likely to overestimate erosion rates. Given transport as the limiting process, the so called erodibility value, estimated with those models, is indeed sediment transportability under the study condition. The effects of slope length on interrill erosion regimes need to be studied further under a wider range of conditions.

Abiotic processes are insufficient for fertile island development: A 10‐year artificial shrub experiment in a desert grassland

AbstractThe relative importance of biotic and abiotic processes in the development of “fertile islands” in dryland systems has rarely been investigated. Here we approached this question by using artificial shrubs, which exclude plant litter production and soil nutrient uptake, but retain the functions of trapping windblown material, funneling of stemflow, and differential rain splash. We conducted a vegetation manipulation study more than a decade ago in the desert grassland of southern New Mexico and subsequently revisited the site in 2012 and 2015. The results show that no notable soil mounds were observed under the artificial shrubs; however, soil texture under the artificial shrubs has gradually changed to resemble the patterns of soil particle‐size distribution under natural shrubs. Our results highlight that with the exclusion of direct biotic additions, soils captured by shrub canopies are not necessarily fertile and thus do not themselves contribute to the development of fertile islands.

Developing a Parameterization Approach for Soil Erodibility for the Rangeland Hydrology and Erosion Model (RHEM)

Soil erodibility is a key factor for estimating soil erosion using physically based models. In this study, a new parameterization approach for estimating erodibility was developed for the Rangeland Hydrology and Erosion Model (RHEM). The approach uses empirical equations that were developed by applying piecewise regression analysis to predict the differences of erodibility before and after disturbance (i.e., wildfire, prescribed fire, and tree encroachment) and across a wide range of soil textures as a function of vegetation cover and surface slope angle. The approach combines rain splash, sheet flow, and concentrated flow erodibilities into a single parameter for modeling erodibility in most cases. We evaluated the new approach for sites representing different degrees of disturbance associated with burning and tree encroachment. Our results show that the new erodibility approach in RHEM predicts erosion at the plot scale with a satisfactory range of error in all cases. The new approach extends the applications of RHEM to a greater scope of landscapes and soil texture.

The elements and richness of particle diffusion during sediment transport at small timescales

AbstractThe ideas of advection and diffusion of sediment particles are probabilistic constructs that emerge when the Master equation, a precise, probabilistic description of particle conservation, is approximated as a Fokker–Planck equation. The diffusive term approximates nonlocal transport. It ‘looks’ upstream and downstream for variations in particle activity and velocities, whose effects modify the advective term. High‐resolution measurements of bedload particle motions indicate that the mean squared displacement of tracer particles, when treated as a virtual plume, primarily reflects a nonlinear increase in the variance in hop distances with increasing travel time, manifest as apparent anomalous diffusion. In contrast, an ensemble calculation of the mean squared displacement involving paired coordinate positions independent of starting time indicates a transition from correlated random walks to normal (Fickian) diffusion. This normal behavior also is reflected in the particle velocity autocorrelation function. Spatial variations in particle entrainment produce a flux from sites of high entrainment toward sites of low entrainment. In the case of rain splash transport, this leads to topographic roughening, where differential rain splash beneath the canopy of a desert shrub contributes to the growth of a soil mound beneath the shrub. With uniform entrainment, rain splash transport, often described as a diffusive process, actually represents an advective particle flux that is proportional to the land surface slope. Particle diffusion during both bedload and rain splash transport involves motions that mostly are patchy, intermittent and rarefied. The probabilistic framework of the Master equation reveals that continuous formulations of the flux and its divergence (the Exner equation) represent statistically expected behavior, analogous to Reynolds‐averaged conditions. Key topics meriting clarification include the mechanical basis of particle diffusion, effects of rarefied conditions involving patchy, intermittent motions, and effects of rest times on diffusion of tracer particles and particle‐borne substances. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Disease cycle of Stagonosporopsis tanaceti in pyrethrum plants

Pyrethrum (Tanacetum cinerariifolium) is commercially cultivated in Australia for the production of pyrethrins. Ray blight caused by Stagonosporopsis tanaceti is considered a major limiting factor for pyrethrum production. Histopathology of infected pyrethrum plants identified the importance of crown infection where the infection hyphae were confined to the parenchyma cells of the epidermis, hypodermis and cortex without infecting the vascular tissues. Throughout the growth of pyrethrum plants, no vascular infection occurred in the petioles, flower stems and crowns. In infected leaf petioles and flower stems, pycnidia and extra-cellular anthocyanin material developed within 1–2 weeks of infection. The symptoms of infection included a dark brown discolouration or necrosis of infected cells caused by intra- or intercellular colonisation of hyphae. Secondary infection of leaves, flower stems and flower buds most likely occurred through wind, rain or water splash of pycnidiospores released from pycnidia in the leaves and petioles. Roots of the infected plants remained uninfected throughout the growth of the plants. Detailed understanding of the disease cycle of ray blight disease in pyrethrum plants will enable the development of more efficient, targeted control measures.

Honeybees can spread Colletotrichum acutatum and C. gloeosporioides among citrus plants

Postbloom fruit drop (PFD) is an important citrus disease that causes up to 100% yield losses during years in which conditions are favourable for the occurrence of epidemics. The conidia of Colletotrichum acutatum and C. gloeosporioides, causal agents of PFD, are predominantly dispersed by rain splash. At the beginning of epidemics, the distribution of diseased plants is random and the disease progress rate is very high, which is unusual for pathogens spread by rain splash. As the pathogen produces abundant conidia on diseased petals, pollinating insects may contribute to disease dispersal. This study investigated honeybees (Apis mellifera) as dispersal agents of C. acutatum and C. gloeosporioides among citrus plants. Two experiments were carried out in a screenhouse in which citrus plants were protected (or not) in insect‐proof cages. The source of inoculum was placed on one side of the screenhouse, and a honeybee hive was placed on the opposite side. All uncaged plants showed symptoms of the disease, and none of the caged plants exhibited PFD symptoms. The monomolecular model showed a good fit to disease progress in both experiments. Conidium‐like structures of Colletotrichum spp. were identified attached to the bodies of the honeybees by scanning electron microscopy. These results have revealed that honeybees disperse Colletotrichum among citrus plants.

Fine-scale spatial genetic structure of a fungal parasite of coffee scale insects

The entomopathogenic fungus Lecanicillium lecanii persists in a highly dynamic network of habitat patches (i.e., a metapopulation) formed by its primary host, the green coffee scale Coccus viridis. Lecanicillium lecanii is an important biological control of both C. viridis and the coffee rust, Hemileia vastatrix. Successfully managing this biocontrol agent will depend on an increased understanding of the characteristics of its dispersal, as migration between occupied and unoccupied patches is essential for the persistence of this metapopulation. In the present study, we employ a population genetics approach, and show that in our study system, a coffee farm in the Soconusco region of southern Mexico, L. lecanii is characterized by clear spatial genetic structure among plots within the farm but a lack of apparent structure at smaller scales. This is consistent with dispersal dominated by highly localized transport, such as by insects or rain splash, and less dependence on longer distance dispersal such as wind transport. The study site was dominated by a few multi-locus microsatellite genotypes, and their identities and large-scale locations persist across both study years, suggesting that local epizootics (outbreaks) are initiated each wet season by residual propagules from the previous wet season, and not by long-distance transport of propagules from other sites. The index of association, a measure of linkage disequilibrium, indicates that epizootics are primarily driven by asexual, clonal reproduction, which is consistent with the apparent lack of a teleomorph in the study site and the presence of only a single mating type across the site (MAT-1-2-1). Although the same predominant clonal genotypes were found across years, a drastic difference in genotypic diversity was witnessed across two sites between the two years, suggesting that interclonal selection was occurring. In light of the dispersal limitation of L. lecanii, spatial structure may be an essential axis of management to ensure the persistence of L. lecanii and preserve the ecosystem services provided by this versatile biocontrol agent in this and similar coffee farms.

Are rubber-based agroforestry systems effective in controlling rain splash erosion?

In order to evaluate the influence of different types of rubber-based agroforestry systems on soil erosion processes, rainfall and throughfall erosivity (splash erosion potential) were measured in an open field environment and under different vegetation types using sand-filled Tübingen splash cups. Our results indicate that the splash erosion potential under rubber monoculture was, on average, 3.12 times greater than those in the open environment. Splash erosion potential under agroforestry systems was higher than that of an open environment (ranging from 1.22 to 2.18 times greater), except for the rubber and tea system (0.87 times the open environment). However, in all but one system (the rubber and orange system), there was a significant reduction in splash erosion beneath multiple canopies compared to monoculture, especially for the rubber and tea system (0.27 times the monoculture) where it had high sub-canopy closure and low sub-canopy height. The erosion potential under the forest is closely related to the forest structure, especially height and canopy cover. These results indicate that low canopy height with high sub-canopy coverage is the major control on the amount of splash erosion, regardless of how the splash potential is increased by the canopy above. These results highlight the importance of selecting low near-surface intercrops for constructing rubber-based agroforestry systems. This also accentuates the importance of an intact litter layer in rubber plantations to protect the soil against splash erosion. Disturbance of these forests by latex tapping activities, herbicide application and removal of the litter layer during fertilization, for example, will also lead to higher actual splash erosion rates inside the forests in comparison with the open environment.

Report of frozen equine embryos transfer in the Czech Republic

Several explanations have been proposed for the temporal differences in geologic processes associated with the modification of martian impact craters, which occurred throughout the Noachian, and the formation of valley networks, which occurred during the Noachian/Hesperian transition. Here we show that it could be a result of the changing nature of rainfall as the primordial atmospheric pressure on Mars waned through time. We calculate the terminal velocity and resulting kinetic energy from raindrops > 0.5 mm in diameter that would impact the surface of Mars in a CO2-rich atmosphere ranging in pressure from 0.5 to 10 bars. Our analyses indicate that the primordial atmosphere of Mars could not have exceeded ∼4.0 bars as raindrop sizes would have been limited to < 3 mm and surface erosion from rain splash and subsequent crater modification would not have occurred. At pressures between ∼3.0 and 4.0 bars, sediment transport from rain splash could occur, but surface runoff would have been limited, which could explain the modification of impact craters. Once atmospheric pressures waned to ∼1.5 bars, rainfall intensity could begin to exceed the infiltration capacity of most soils, which would be necessary to initiate martian valley network formation. Due to the lower gravity, a storm on Mars that occurred in a 1 bar atmosphere could generate raindrops with a maximum diameter of ∼7.3 mm compared to 6.5 mm on the Earth. However, rainfall from such a storm would be only be ∼70% as intense on Mars, primarily due to the lower martian gravity and resulting lower terminal velocities of the rain drops.

The effect of rain, wind-driven rain and wind on particle transport under controlled laboratory conditions

Transport of soil particles prior to the occurrence of overland flow is one of the big question marks in soil erosion studies. The exact measurement of short-distance transported soil particles is a challenge to soil erosion science due to the particular requirements of the experimental design and test procedure. To quantify amount and distance of each type of transport, we developed an experimental laboratory setup including a multiple-gutter system and the Trier Portable Wind and Rainfall Simulator (PWRS). Measured were amount and travel distance of soil particles detached and transported by raindrops (splash-creep and splash), wind-driven rain (splash-creep, splash-saltation and splash-drift) and wind (reptation and saltation). The test setup included three different agents of erosion (rain solely, wind-driven rain and wind solely), two substrates (sandy and loamy), three surface structures (smooth/ grain roughness, rills lengthwise and rills transversal) and three slope angles (0°/horizontal, 7° downslope and 7° upslope). The results give detailed transport patterns of the three erosion agents under the varying substrates and surface conditions up to a distance of 1.6m. Influence of the surface factors varies, whereas the factor “agent of erosion” seems to be the most crucial one. Under the applied rain intensity and wind velocity, wind-driven rain splash generates the highest erosion and a further travel distance of the particles due to the combined action of wind and rain. The impact of all three agents of erosion implicates considerable redistribution processes and is a crucial factor for investigation of source- and sink dynamics, field scale sediment budgets and connectivity.

Population Genetic Structure of QoI-Resistant Pestalotiopsis longiseta Isolates Causing Tea Gray Blight

Resistance to the quinone outside inhibitor (QoI) fungicides in the tea gray blight-causing fungus Pestalotiopsis longiseta is a serious problem in Japanese tea cultivation. We conducted a population genetic analysis of QoI-resistant P. longiseta isolates on the Makinohara Plateau, Shizuoka Prefecture, Japan's largest tea-growing area, to elucidate the disease's epidemiology and the spread of QoI resistance. Inter simple sequence repeat (ISSR) analysis of 1,083 isolates from 395 fields collected from 2009 to 2012 detected 42 ISSR types, designated as PL01 to PL42. A total of 18, seven, and 38 ISSR types were detected in highly resistant, moderately resistant, and sensitive isolates, respectively. No distinct phylogenetic relationship corresponding to QoI sensitivity or sampling location was observed. No annual changes in the population genetic structure of highly resistant isolates were observed during the study period. A different ISSR type was predominant among QoI-resistant isolates in each region. Analysis of molecular variance revealed significant genetic differentiation in populations of highly resistant isolates among regions (FCT = 0.213) and farmers (FCT = 0.071). Consequently, we speculate that QoI-resistant P. longiseta strains occurred in a number of clonal lineages and spread by both natural and artificial transmission, such as rain splash and plucking machines, throughout each region on the Makinohara Plateau.

A reconsideration of natural dispersal distances of the pine pathogen Dothistroma septosporum

The natural spread of Dothistroma septosporum, the causal agent of a foliar disease of pines, was investigated at three sites in the south of England using trap plants. The pathogen is considered to be primarily rain‐splash dispersed, but this study shows that it can be spread many hundreds of metres from an inoculum source, demonstrating that dispersal is not solely via rain splash. The maximum distance the pathogen was recorded from any infection source was in excess of 1400 m, over five times the distance defined in the only previous work of this kind. Consequently, a reassessment of forest and production nursery management practices is called for, as these assume that the pathogen only spreads naturally over limited distances. Detection of the pathogen on trap plants over 100 m from the inoculum source was, in most cases, only possible using quantitative real‐time PCR diagnosis. The entire diagnostic procedure, from DNA extraction to amplification, was able to detect a minimum of approximately 17 D. septosporum cells in a pine needle sample, assuming only a moderate DNA extraction efficiency of 30%.

Limits on the morphogenetic role of rain splash transport in hillslope evolution

AbstractAn experimentally parameterized equation for rain splash transport was generalized in order to compute annual sediment transport rates for a range of climates. The resulting equation was then used to examine the conditions under which rain splash transport can play a significant morphogenetic role in water‐eroded landscapes. Rain splash can generate convex hillslopes on short, steep hillslopes such as the margins of gullies in unconsolidated sediments. However, comparison of predicted profiles with long, convex hillslopes in postorogenic landscapes indicates that rain splash cannot transport sufficient quantities of sediment to produce the observed convex hillslope profiles. Rates of biogenic transport estimated from the literature also appear to be insufficient. Sheetwash transport appears to be the only process capable of molding these extensive convex hillslope profiles despite the common assumption that it generates exclusively concave forms.

Illuminating wildfire erosion and deposition patterns with repeat terrestrial lidar

AbstractErosion following a wildfire is much greater than background erosion in forests because of wildfire‐induced changes to soil erodibility and water infiltration. While many previous studies have documented post‐wildfire erosion with point and small plot‐scale measurements, the spatial distribution of post‐fire erosion patterns at the watershed scale remains largely unexplored. In this study lidar surveys were collected periodically in a small, first‐order drainage basin over a period of 2 years following a wildfire. The study site was relatively steep with slopes ranging from 17° to &gt; 30°. During the study period, several different types of rain storms occurred on the site including low‐intensity frontal storms (2.4 mm h−1) and high‐intensity convective thunderstorms (79 mm h−1). These storms were the dominant drivers of erosion. Erosion resulting from dry ravel and debris flows was notably absent at the site. Successive lidar surveys were subtracted from one another to obtain digital maps of topographic change between surveys. The results show an evolution in geomorphic response, such that the erosional response after rain storms was strongly influenced by the previous erosional events and pre‐fire site morphology. Hillslope and channel roughness increased over time, and the watershed armored as coarse cobbles and boulders were exposed. The erosional response was spatially nonuniform; shallow erosion from hillslopes (87% of the study area) contributed 3 times more sediment volume than erosion from convergent areas (13% of the study area). However, the total normalized erosion depth (volume/area) was highest in convergent areas. From a detailed understanding of the spatial locations of erosion, we made inferences regarding the processes driving erosion. It appears that hillslope erosion is controlled by rain splash (for detachment) and overland flow (for transport and quasi‐channelized erosion), with the sites of highest erosion corresponding to locations with the lowest roughness. By contrast, in convergent areas we found erosion caused by overland flow. Soil erosion was locally interrupted by immobile objects such as boulders, bedrock, or tree trunks, resulting in a patchy erosion network with increasing roughness over time.

First Report of Sclerotium rolfsii Associated with Boll Rot of Cotton in India

Cotton (Gossypium hirsutum) is the most important fiber-yielding crop grown in India and worldwide. In India, during 2012-2013, 1.16 million ha were under cotton cultivation, with an average production of 480 kg/ha. As production of cotton has increased in areas, so has the occurrence of various pests and diseases. During a field survey conducted in the cotton-growing areas of Devalapura Village of the Mysore District during August and September 2013, a boll rot disease was observed with distinctive symptoms and signs of a fungal pathogen observed in 27% of fields. Incidence of the boll rot within fields ranged from 8 to 12%, while 3 to 5% stem rot was observed on boll rot-affected plants in about 15 to 18 ha of total cotton fields surveyed. White mycelial strands with spherical, dark-brown sclerotial bodies were found on the surface of infected bolls near the ground and the affected bolls were completely decayed. The disease-affected bolls were disinfested with 2% sodium hypochlorite solution for 3 min, and 5-mm samples were placed on potato dextrose agar (PDA) medium and incubated at 28 ± 2°C. The fungal colonies that grew from infected tissues were whitish with cottony aerial mycelium and abundant globoid sclerotia observed after 10 to 12 days of incubation. Sclerotial bodies were initially pale whitish, becoming dark brown over time. Based on morphological and cultural characteristics, the associated fungal pathogen was identified as Sclerotium rolfsii Sacc. The identity of the pathogen was further confirmed through PCR amplification of internally transcribed spacer (ITS) region using ITS1/ITS4 universal primer. The amplified PCR product (550 to 570 bp) was sequenced and BLASTn search comparison revealed 100% homology with Sclerotium rolfsii (Accession Nos. JF966208 and JX914480). A representative sequence of S. rolfsii was deposited in GenBank (KP412471.1 and KP412472.1). Further, pathogenicity tests were conducted on 30 healthy cotton bolls in vivo by inoculating the bolls with sclerotia obtained from 12-day-old cultures; noninoculated bolls served as controls. Characteristic boll rot symptoms were observed on 60% of total inoculated cotton bolls 7 days after inoculation and S. rolfsii was reisolated from the infected bolls and the identity of the pathogen was confirmed. S. rolfsii is a serious soil-borne fungal pathogen with a wide host range (Mullen 2001) and prevalent in tropical and subtropical regions, where high temperature and moisture are sufficient to permit growth and survival of the fungal pathogen (Punja 1985). The association of S. rolfsii with boll rot of cotton has been reported from Bangladesh (Shamsi et al. 2008). In India, major diseases of cotton include wilt (Fusarium oxysporum f. vasinfectum), anthracnose (Colletotrichum capsici), and areolate mildew (Ramularia areola) (Rangaswamy and Mahadevan 2002). S. rolfsii infect bolls on branches near ground level, causing boll decay. Numerous sclerotia produced on the affected bolls are the source of secondary inoculum and are dispersed by rain splash during the late monsoon, thus resulting in new infections on the basal stem and leaves of the cotton plant. The disease has resulted in reduced yield and market value of the crop. To the best of our knowledge, this is the first report on the occurrence of S. rolfsii causing boll rot of cotton in India.

Influence of Temperature and Humidity on the Viability of Ophiognomonia clavigignenti-juglandacearum Conidia.

Butternut canker, caused by the fungus Ophiognomonia clavigignenti-juglandacearum, primarily kills butternut (Juglans cinerea). Rain splash and local air currents are the primary means of conidia dispersal but that does not explain its long-distance spread and infection of isolated trees. Dispersal by insect or animal vectors or plant material likely necessitates the ability for conidia to tolerate drying for a period of time over variable temperature and humidity conditions. The objective of this study was to determine the influence of temperature and humidity on conidial germination and survival of air-dried conidia. Conidia collected from 1-month-old cultures germinated on water agar over a wide range of temperatures (4 to 32°C) and were viable after brief periods at 36°C when returned to a lower temperature. Viability of air-dried conidia held on nylon membranes at various temperatures and humidities varied from less than a day at 28°C and 90% relative humidity (RH) to a mean of 15 days at 20°C and 80% RH. RH had the least effect on viability at 12°C, with conidia remaining viable for 7 days at most humidity levels tested. Conidia held at 100% RH began germinating on the membranes after 21 days. Conidia in a water suspension remained viable for 168 days at all temperatures tested. These results suggest that O. clavigignenti-juglandacearum conidia may remain viable on the surface of a vector or plant material and seed for over 2 weeks, given the proper conditions, or for much longer if in water or in an environment of saturated humidity. This potential may, in part, explain the frequent presence of the disease on isolated trees.