Aphid abundance and potato virus y transmission in imidacloprid-treated potatoes

Imidacloprid is a commonly used insecticide that is highly effective in controlling insect pests of potato. The relatively high cost of imidacloprid provides commercial growers with a strong incentive to reduce field application rates. In the present study, we investigated the impact of reduced imidacloprid rates on aphid abundance and potato virus Y transmission within potato fields. Infurrow applications of imidacloprid provided better aphid control even at a reduced rate when compared to threshold-based foliar applications, but did not suppress the spread of the non-persistently transmitted potato virus Y. Virus transmission was significantly decreased in foliar-sprayed plots, possibly because foliar applications incapacitated aphid vectors probing or feeding on infected plants. Despite these generally encouraging results, we still observed 2.3 to 2.7 fold increase in virus infection at foliar-treated plots at the end of the growing season.     

Integrated pest management in field vegetable crops in northern Europe — with focus on two key pests

Improvements in (1) insecticide application, (2) supervised control, and (3) pest forecasting systems have each helped to reduce considerably the amounts of insecticides required to control fly, caterpillar and aphid infestations in vegetable crops in northern Europe. By growing plants that are partially resistant to certain major pests, it is now possible to apply even less insecticide than the dose recommended for the crop. In crops where only small amounts of insecticides are applied, natural predators should prevent large increases in pest insect populations and natural parasitoids should reduce the numbers of pest insects entering subsequent generations. The possible impact of introducing transgenic plants and the use of physical (crop covers), cultural (crop rotation, undersowing) and microbial (e.g. fungi, bacteria and nematodes) methods of control are also discussed. The withdrawal of certain insecticides, as a result of environmental and commercial pressures, means that some crops may soon be without appropriate insecticides for controlling one or more of the major pest species. Whether such systems will be sustainable, remains to be seen.     

Integration of soil applied neonicotinoid insecticides and acibenzolar-S-methyl for systemic acquired resistance (SAR) control of citrus canker on young citrus trees

Soil application of systemic neonicotinoid insecticides for control of psyllid vectors of Huanglongbing disease on young citrus trees also produces season-long SAR control of citrus canker caused by Xanthomonas citri subsp. citri Schaad et al. The neonicotinoids imidacloprid (IMID) and thiamethoxam (THIA) were compared with soil or sprinkler applications of the commercial SAR inducer acibenzolar-S-methyl (ASM) and foliar sprays of copper hydroxide (CH) and/or streptomycin (STREP) to evaluate their effects on the percentage of canker-infected leaves on 2-yr-old ‘Vernia’ orange and 3-yr-old ‘Ray Ruby’ grapefruit trees in Southeast Florida. All treatments significantly reduced the incidence of foliar canker compared to the untreated check. Soil drenches of ASM and season long rotations with IMID and THIA were highly effective for suppressing foliar canker on young grapefruit and orange trees under weather conditions absent of high intensity rains or tropical storms. Sprinkler application of ASM was less effective than soil drench. The level of control for SAR treatments was comparable to eleven 21-day interval sprays of CH and/or STREP. SAR induced by soil-applied insecticides provides substantial benefits for canker disease management on young citrus trees that may be augmented with ASM.     

Aphidophagous Parasitoids can Forage Wheat Crops Before Aphid Infestation,Parana State,Brazil

Aphid parasitoids are common in Brazilian wheat fields, and parasitize aphids at the wheat tillering stage. However, there is little information available about when this natural enemy occurs in wheat crops. This study investigated the initial occurrence of aphid parasitoids in four commercial wheat crops in northern Paraná during the 2009 crop season. We installed two Malaise traps at each wheat farm, and 400 tillers were assessed weekly in each field for aphid abundance. During this study, we captured 4,355 aphid parasitoids and 197 aphids. Three species of braconid parasitoids were identified, including Aphidius colemani (Viereck 1912), Lysiphlebus testaceipes (Cresson 1880), and Diaeretiella rapae (McIntosh 1855). The aphids species identified were Rhopalosiphum padi (Linnaeus 1758) and Sitobion avenae (Fabricius 1775). This study showed that aphid parasitoids are present in wheat crops even when aphid densities are low, and in one farm, occurred before the aphids colonization. These reports can justified the high efficiency of these natural enemies against aphids in wheat fields.     

Population development of Phorodon humuli and predators (Orius spp.) within hop cones: Influence of aphid density on hop quality

The price of hops in Spain is given per kilogram of dry product and according to their content of alpha-acids. If any aphids are noted inside cones there is a penalty of up to 10% of the dry weight. Experiments were performed in a hop garden in 2002, 2003 and 2004 to investigate population development of the damson-hop aphid, Phorodon humuli, and its natural enemies in hop (Humulus lupulus) cones. Furthermore, we investigated if the alpha-acid content was affected by aphid populations. Aphids were first observed within hop cones at the beginning of August and numbers rose sharply from mid- to late August onwards. Aphid infestation of cones reflected the numbers observed on foliage in July, with a greater population halfway up the bines (3 m height) than at the top (6 m height). The alpha-acid content of dried hops was unaffected by the aphid population on leaves, but aphid contamination reduced the economic value of the crop because of arbitrary commercial criteria related to the presence of aphids in cones. The hop aphid’s main natural enemies within cones were anthocorids, all belonging to the genus Orius. The anthocorid population in cones grew in tandem with the aphid population, suggesting that these predators are worthy of consideration in an integrated management approach.     

Growing seed potatoes in the Sinai Desert

Summary Seed potatoes grown during winter in the Sinai Desert (Sharm el Sheik) yielded in autumn significantly more (17%) than commercial seed grown in spring. Seeds grown in winter in an agricultural area at Yotvata yielded similarly to the commercial controls. Almost no spread of PVY or PLRV was found at Sharm probably because of the very low alate population there, whereas at Yotvata the incidence of PLRV was over 40% probably as a consequence of a high aphid populations during the growing season. Alate populations were also observed in the Sinai Desert at A-tur, a completely arid area, and at Firan, an isolated oasis where agriculture is practised. At A-tur only a few alatae were trapped, but large numbers were found at Firan, especially starting in March, but not during winter the proposed season for growing seed potatoes. The possibility of growing high quality seed potatoes at Sharm is discussed. Contribution No 107, 1980 series, Volcani Center.     

Interactions between crop biomass and development of foliar diseases in winter wheat and the potential to graduate the fungicide dose according to crop biomass

Foliar pathogens such as Zymoseptoria tritici and Puccinia striiformis causing septoria leaf blotch and yellow rust respectively can cause serious yield reduction in winter wheat production, and control of the diseases often requires several fungicide applications during the growing season. Control is typically carried out using a constant fungicide dose in the entire field although there may be large differences in crop development and biomass across the field. The objective of the study reported in this paper was to test whether the fungicide dose response curve controlling septoria leaf blotch and other foliar diseases in winter wheat was dependent on crop development and biomass level. If such a biomass dependent dose response was found it was further the purpose to evaluate the potential to optimize fungicide inputs in winter wheat crops applying a site-specific crop density dependent fungicide dose. The study was carried out investigating fungicide dose response controlling foliar diseases in winter wheat at three biomass densities obtained growing the crop at three nitrogen levels and using variable seed rates. Further the field experiments included three fungicide dose rates at each biomass level, an untreated control, and 75%, 50% and 33% of the recommended fungicide dose rate and the experiments were replicated for three years. Crop biomass had a significant influence on occurrence of septoria and yellow rust with greater disease severity at increasing crop biomass. In two of three years, the interaction of crop biomass and fungicide dose rate had a significant influence on disease severity indicating a biomass-dependent dose response. The interaction occurred in the two years with high yield potential in combination with severe disease attack. If the variation in crop density and biomass level obtained in the study is representative of the variation found cultivating winter wheat in heterogeneous fields, then there seems to be scope for optimizing fungicide input against foliar diseases site-specific adapting the dose according to crop density/biomass.     

Effectiveness of Combined Use of Mineral Oil and Insecticide Spray in Reducing <Emphasis Type="Italic">Potato Virus Y</Emphasis> (PVY) Spread under Field Conditions in New Brunswick,Canada

In the 2014 and 2015 crop seasons, the efficacies of different types, rates and combinations of mineral oil and insecticide foliar sprays for reducing Potato virus Y (PVY) spread were tested in controlled field trials in New Brunswick (NB), Canada. Experimental plots were planted with certified PVY-free Goldrush, supplemented with known virus-infected seed to raise PVY inoculum to 2.3% and 3% at the beginning of the 2014 and 2015 seasons, respectively. Treatments consisted of mineral oil-only sprays at different application rates, insecticide-only sprays of differing numbers, and several combined mineral oil and insecticide spray regimes, all compared to a no-spray control treatment. PVY spread to 18% (2014) and 22% (2015) of initially virus-free plants in no-spray control plots, with significant reductions observed in PVY spread in several treatments. Greatest PVY reductions, as low as 4% (2014) and 12% (2015), were in combined mineral oil and insecticide spray treatments, followed by oil-only sprays; while insecticide-only sprays did not significantly reduce PVY spread. As well as measuring PVY spread to marked test plants and randomly collected post-harvest tuber sample from the plots, exhibited similar treatment pattern for PVY incidence. Multiple logistic regression modeling confirmed the relative efficacy of combined oil and insecticide sprays for reducing PVY spread, while accounting for variable inoculum and aphid factors. Modeling also highlighted the importance of planting low-PVY seed initially, and of early application of foliar sprays. Local best management practice recommendations for reduction of in-field PVY spread were discussed.     

Potato (Solanum tuberosum L.) cultivar response to bentazon and crop oil

Bentazon [3-(l-methylethyl)-(1H)-2,l,3-benzothiadiazin-4(3H)-one 2,2-dioxide] was applied at two rates to nine potato cultivars in the field, with or without a crop oil, to plant foliage at two different heights. Seven days after application of bentazon, foliar injury ranged from 0 to 36 percent depending on rate, cultivar, and plant height at the time of application. Of nine cultivars tested, Russet Burbank was the most susceptible to injury and Red LaSoda the least susceptible. A bentazon application rate of 1.12 kg a.i./ha, compared to a rate of 0.84 Kg a.i./ha, resulted in a biologically insignificant increase in foliar injury of two to four percent and a significant increase in weed control. The addition of crop oil to bentazon did not result in an increase of foliar injury, but increased the percentage of weeds controlled approximately two-fold. With one exception, application of bentazon to plants 8–15 cm in height resulted in less foliar injury than application to plants 20–30 cm in height. Application of bentazon to larger plants resulted in lower yield and quality. For most cultivars, the application of bentazon (1.12 kg/ha a.i.) and crop oil (2.3 L/ha) to 8 to 15 cm-tall potato plants resulted in no appreciable loss in potato tuber yield, while controlling 74–85 percent of broadleaf weeds.     

Peach orchard protection strategies and aphid communities: Towards an integrated agroecosystem approach

Aphid populations, including a widespread pest, Myzus persicae, are known to be affected by cultural practices, plant growth and natural regulation, but their control in orchards most frequently relies on regular treatments aiming to avoid direct damage and virus transmission. This study aimed first at characterising farmers’ management strategies towards aphids and, secondly evaluating them in terms of efficiency and impact on aphid natural enemies. Protection methods were first identified through semi-open interviews and peach orchard pesticide records from 19 organic and conventional fruit farms in south-eastern France. It appeared that in addition to the regular use of direct control, farmers also used cultural and/or alternative methods and various decision-making indicators to optimise their orchard crop protection. Comprehensive variables based on the technical guidelines of the IOBC (International Organisation for Biological and Integrated Control) for crop protection made it possible to differentiate four strategies: intensive, optimised, input-substitution and integrated. Visual monitoring of aphid infestations and of beneficial populations was performed at the plot level. Brachycaudus schwartzi and Myzus varians were the most frequently encountered aphid species but the structures of the communities were affected by protection strategies: the more integrated the strategy, the more diverse and abundant the aphids. The two most effective strategies were dominated by chemical treatments, whereas the other two, less detrimental to aphid antagonists, were mostly carried out by organic farmers. Variations in aphid communities were related to: (i) the use of toxic products, correlated with low-infestations and low-abundance and diversity of antagonists; (ii) alternative and cultural methods (such as mowed ground cover and manual pruning of infested branches, respectively) that promoted high populations of both guilds. Unexpectedly, the structures of these communities were not related to the number of treatments, management of tree vigour or of the surrounding environment. The four strategies identified can be further interpreted as steps towards a redesign of orchard protection where integrated organic farming would be the most advanced stage. However, the paradox between effective protection strategy and biodiversity conservation still remains, casting doubts on the agroecological concept of sustainability.     

喷施铁肥对不同基因型小麦产量和铁吸收的影响

为研究小麦的最佳喷施铁肥时期并筛选高铁基因型小麦,采用盆栽方法,以14个不同基因型小麦为材料,通过不同时期喷施铁肥,比较其对小麦产量和铁吸收的影响。结果表明,小麦拔节期叶面喷施铁肥可以增加籽粒产量、籽粒铁含量、籽粒铁累积量及占植株总铁积累量的比例。拔节期喷施铁肥提高了小麦根系的铁含量、累积量及根系铁占植株总铁累积量的比例。不同基因型小麦喷施铁肥时茎中的铁含量、铁累积量及占植株总铁累积量的比例均显著降低。小麦体内的铁主要储存在根系中,地上部的铁主要分布在叶片等营养器官中。与低铁基因型相比,高铁基因型小麦具有较低的籽粒产量和生物量,但籽粒产量占全株总生物量的比例却高于低铁基因型。无论是否喷施铁肥,高铁基因型小麦籽粒铁含量、铁累积量及占植株总铁累积量的比例均显著高于低铁基因型小麦。无论是否喷施铁肥,高铁基因型的根系生物量显著低于低铁基因型,但根系铁含量则显著高于低铁基因型,铁累积量则没有显著差异。拔节期喷施铁肥是促进不同基因型小麦生长、改善小麦铁营养状况的有效途径。     

Impact of pesticide seed treatments on aphid control and yield of wheat in the Sudan

Mixtures of imidacloprid and tebuconazole, were evaluated for three consecutive growing seasons, to determine the effects on plant stand, aphid control and wheat grain yield. At rates of 1.05/0.04 and 0.7/0.04 g a.i of pesticide, respectively, per kg of seeds, plant stand per unit area increased compared with their respective untreated control. Both rates of imidacloprid efficiently controlled the maize aphid (Melanaphis maidis) and suppressed the green bug (Schizaphis graminum) for 6–8 weeks after sowing. There were substantial differences among the different treatments in the number of grains/ear and the 1000-grain weight. These differences were reflected in 90% and 30% average increase in the total grain yield of the wheat crop raised from seeds treated with the mixture relative to the corresponding untreated control and a standard mixture of lindane plus thiram, respectively. This strategy of using imidacloprid as seed dressing allowed easy application, gave adequate reliable control of aphids and less hazardous to the environment.     

Pest intensity-crop loss relationships for the leafminer fly Liriomyza huidobrensis (Blanchard) in different potato (Solanum tuberosum L.) varieties

Quantifying the relationship between plant injury and yield loss is a critical component for decision making in integrated pest management (IPM). The effects of foliar injury during different growth stages of potato (Solanum tuberosum L.), caused by natural populations of the leafminer fly Liriomyza huidobrensis (Blanchard) on potato yield, and the economic injury level for potato varieties with different maturity times were studied in the Cañete valley on the central coast of Peru. Field experiments were carried out during the potato cropping season (July–November) in the years 2005, 2007 and 2008 using early, middle-late and late potato varieties. Insecticides with different control efficiencies or the use of varying numbers of insecticide applications obtained various levels of leafminer fly infestation intensities. Leafminer fly infestation caused foliar injury in all potato varieties resulting in yield losses, which varied, according to the seasonal adult flight activity in each year of evaluation, between 22% in 2007 (2508 adults/trap/season) and 51% in 2005 (3147 adults/trap/season). Foliar injury increased with the development and growth of the potato plant, with the highest injury registered at the end of the cropping period. Early varieties showed higher foliar injury and yield loss than late potato varieties with the exception of Maria Tambeña, which is considered partially resistant to leafminer fly infestation. Yield losses increased linearly by each percentage of foliar injury in all potato varieties. Critical point and multiple point models made it possible to identify the potato growth stage at which leafminer infestation had the greatest effect on yield. Pest infestation-crop loss relationships indicated that the accumulated foliar injury up to the growth stages of flowering and berry formation produced the highest yield losses in the different potato varieties. Economic injury levels varied according to control costs and commodity values in each potato variety (Desiree: 21–28%, Revolucion: 34–47%, Canchan: 31–40%, Maria Tambeña: 40–53%, Tomasa: 55–74%, and Yungay: 40–54% of foliar injury). Potato varieties can tolerate considerable levels of foliar injury by the leafminer fly before control measures are needed. The use of the economic injury levels is suggested as a decision support tool for managing the leafminer fly at reduced pest management costs.     

Analysis of yield-attributing traits for high-yielding wheat lines in southwestern Japan

Development of wheat cultivars that achieve high yields despite the short growing season is essential for increasing wheat production in southwestern Japan. The objectives of this study were to assess the genetic progress in grain yield and to clarify yield-attributing traits of high-yielding wheat lines in southwestern Japan. We conducted field experiments for two growing seasons (2012–2013 and 2013–2014) using three commercial wheat cultivars (Shiroganekomugi, Chikugoizumi, and Iwainodaichi) and four high-yielding wheat lines including Hakei W1380 developed in southwestern Japan. In an ancillary field experiment, we compared a commercial cultivar, Shiroganekomugi, and a high-yielding line, Hakei W1380, in the 2014–2015 season. Across the two seasons, grain yield of high-yielding lines was generally higher than commercial cultivars. Hakei W1380 achieved the highest grain yield across the two seasons, and successfully produced more than 900 g m^?2 in the 2013–2014 season. Correlation analysis showed that recent yield progress of wheat lines in southwestern Japan was derived from enhanced biomass production and grain number m^?2. Larger numbers of grains m^?2 in high-yielding lines than in commercial cultivars were associated with higher crop growth rate at the pre-anthesis stage, and therefore higher spike dry weight m^?2 at anthesis. Genotypic differences in crop growth rate from jointing to anthesis resulted mainly from differences in leaf area index. These results indicate that further improvements in grain yield in southwestern Japan could be achieved by increasing the amount of radiation intercepted at the pre-anthesis stage and grain number m^?2.     

Insecticidal control of cereal aphids and its impact on the epidemiology of the barley yellow dwarf luteoviruses

Imidacloprid is a seed-applied nitroguanidine insecticide that has both contact and long-lasting systemic properties. Its direct effects on cereal aphid populations and indirect effects on barley yellow dwarf luteovirus (BYDV) incidence were examined in laboratory and field studies. Adult longevity and fecundity of three aphid species caged on various aged, imidacloprid-treated oat or wheat plants was reduced, although the quantitative efficacy of the compound differed among aphid species. The transmission efficiency of BYDV by aphids to imidacloprid-treated plants was lower, relative to nontreated plants, when plants were 10 days old, but similar to nontreated when plants were 24 days old. In three years of field trials using winter wheat and two years using spring oat, aphid populations were reduced significantly in imidacloprid-treated plots relative to nontreated plots. Although the number of alighting alate aphids did not differ significantly between imidacloprid-treated or nontreated plots, the number of apterous aphids remained significantly lower in the treated plots for the majority of the growing season. BYDV epidemics did not develop in spring oat in either of the two years. BYDV epidemics did develop in the fall in the emerging winter wheat crop. Rhopalosiphum maidis was the predominant aphid migrating into the 1991–1992 crop. The incidence of the BYDV-RMV serotype was three times higher in nontreated plots than in imidacloprid-treated plots. In the 1992–1993 and 1994–1995 seasons, R. maidis and R. padi were the predominant aphids migrating into the crop, although R. padi was the predominant colonizing aphid. The incidence of BYDV-RMV was similar in imidacloprid treated and nontreated plots in 1992–1993, but the incidence of the BYDV-PAV serotype was significantly less in the treated plots. In 1994–1995, the incidence of both BYDV-PAV and BYDV-RMV was higher in the nontreated plots than in the imidacloprid-treated plots. Imidacloprid offers several advantages both in terms of its long-lasting systemic activity and its mode of applicaiton. As a seed treatment, the amount of material applied is minimized, there is little waste and the environmental impacts are reduced. The effectiveness of the compound at reducing the fecundity or reproductive rate of several aphid species may be advantageous in controlling secondary spread of viruses. However, the contact properties of the compound, important in repelling aphids or preventing phloem feeding, may be short-lived, thus reducing its effectiveness in preventing primary spread of the virus into the crop, especially under high aphid and inoculum pressure.     

Temporal sensitivity ofAlternaria solani to foliar fungicides

Alternaria solani Sorauer (Ellis) is a causal agent of early blight of potato (Solanum tuberosum L.). Sensitivity to chlorothalonil, triphenyl tin hydroxide (TPTH), and mancozeb was determined for field isolatesof A. solani in 1998 and 1999. Specifically, the relative sensitivity ofA. solani to foliar fungicides in field populations collected from several commercial fields was evaluated over the course of two growing seasons. Sensitivity to these foliar fungicides was determined with anin vitro spore-germination assay method. The fungicide concentration that inhibited conidial germination by 50% (EC₅₀) was estimated for each isolate. Sensitivity ofA. solani isolates to TPTH and mancozeb varied little during the growing season, possibly because there were few TPTH and mancozeb applications made to the fields studied. At several locations, however, repeated exposure ofA. solani populations to chlorothalonil resulted in considerable variability in sensitivity, frequently causing isolates to have decreased sensitivity to this fungicide at the end of the growing season. In five of seven fields, isolates of A.solani collected at the end of the season were significantly less sensitive to chlorothalonil than isolates collected at the beginning of the season.     

Foliar Symptoms Caused by Potato mop-top virus on Potato Plants During Vegetative Propagation in Scotland and Their Association With Tuber Yield, Spraing and Tuber Infection

The effect of foliar symptomatic infection by Potato mop-top virus (PMTV) on yield of tubers, spraing and infection in daughter tubers, and foliar symptom development and tuber infection in the following generations of propagation was investigated in commercial seed potato crops in Scotland. Six crops covering cvs Atlantic, Hermes, Nicola and Cara were studied between 2000 and 2006 by labelling paired replicates of plants with foliar symptoms and plants with no symptoms. Tubers from plants with no symptoms rarely produced plants with foliar symptoms in the following generation. Plants with no symptoms produced more infected tubers if they had been derived from plants with foliar symptoms the previous year than from plants with no symptoms. The proportion of daughter plants with foliar symptoms produced by tubers from plants with foliar symptoms in year 1 ranged from 19–41% and seemed to be associated with the severity of foliar symptoms. The detection of PMTV by ELISA in samples of leaves from plants with foliar symptoms ranged from 13% for cv. Cara to 59% for a crop of cv. Atlantic in 2004. The amounts of spraing were generally low but tended to be greater for tubers from plants with foliar symptoms than those from plants with no symptoms. These results indicate that roguing plants with foliar symptoms in seed potato crops could achieve an improvement in crop health but might be impractical when diseased plants are too prevalent.     

Movento, an innovative ambimobile insecticide for sucking insect pest control in agriculture: Biological profile and field performance

The tetramic acid derivative spirotetramat (brand name Movento^®), has shown an outstanding performance against sucking insect pests in laboratory and greenhouse assays as well as in semi-field and field trials. The product acts as an inhibitor of lipid biosynthesis and affects juvenile stages with additional effects on adult fecundity. There is no cross-resistance to any other insecticide. After foliar application spirotetramat penetrates through the leaf cuticle and is translocated as spirotetramat-enol via xylem and phloem, up to growing shoots and down to roots. This full ambimobility or two-way systemicity (phloem and xylem transport) ensures the control of hidden and soil living sucking pests after foliar application and protects new shoots. The worldwide field development of spirotetramat in Bayer CropScience AG resulted in numerous uses against many species of whiteflies, aphids, scales (soft and armoured scales), mealy bugs, psyllids and selected thrips species in vegetables, cotton, soybean, pome and stone fruit, grapes, hop, citrus, nut trees and banana. The new mode of action renders spirotetramat as an excellent rotation partner with existing products for the management of aphid, whitefly and psyllid populations, which are frequently resistant to conventional insecticides. Moreover, only low adverse effects have been found on beneficial arthropods, which make the product suitable for modern integrated pest management (IPM) systems. These unique properties contribute to safeguarding the crop yield potential both in quality and quantity. In this paper, the new chemistry is presented, compared with standard insecticides and novel applications in IPM systems are discussed.     

Determinants of groundnut rosette virus disease occurrence in Uganda

Groundnut rosette virus disease (GRVD) is the major constraint to groundnut (Arachis hypogaea) production in Uganda. It is principally transmitted by the groundnut aphid (Aphis craccivora Koch). The disease is known to cause total crop failure in cases where susceptible varieties are used. During any particular season, GRVD displays variations in incidence and severity in different agro-ecologies within the country, but the reasons for the varying disease patterns remain unclear. This study was aimed at establishing the factors influencing the occurrence of GRVD in Uganda. Trials were established for three seasons in four groundnut growing locations situated in different agro-ecologies in Uganda. Four groundnut genotypes were used as treatments in a randomized complete block design with four replications. Disease progress and aphid populations were assessed at 4, 8 and 12 weeks after planting. Data on environmental factors; particularly rainfall, temperature and wind speed were obtained from standard meteorological stations located at/near the study sites. Soil samples and yield data were also obtained in each season. The study revealed that disease incidence; severity and groundnut yields were significantly affected by season, location and genotype. The same applied to their three way interactions. Levels of disease infection were found to be majorly influenced by rainfall and wind speed. Disease incidence and severity were generally higher in conditions with less rainfall and low wind speeds. The Pearson's two tailed correlation between total rainfall and disease incidence for all trial sites was negative and highly significant (r = −0.280, P ≤ 0.01). The same was true for wind speed and disease incidence (r = −0.476, P ≤ 0.01). However, there was no conclusive trend between temperature and disease incidence with the Pearson's two tailed correlation showing significantly positive and negative trends depending on location.     

Comparative analysis of aphid vector behavior in response to potato plants grown from field tubers, minitubers or plantlets

The greater sensitivity to aphid vectored diseases of potato plants originating from plantlets or minitubers compared to plants originating from field grown tubers could not be explained by changes in probing behavior, by increased aphid movement between plants or by greater levels of aphid colonization favorable to virus spread. The number of seconds to first probe, the duration and number of seconds before the long probe and the duration of the long probe were statistically similar on all plant types. The average residency time of green peach aphids on each plant type was also statistically the same. WingedMyzus persicae (Sulzer) (green peach aphid) landed in greater numbers on older than on younger plants regardless of plant type. This suggests that the level of aphid colonization is determined by a combination of factors such as height, surface and age of plants but not by plant origin. Where virus vectors are present early in the season, the greater number of aphids landing on the relatively older and larger transplants may contribute to higher infection levels, but later in the season plant physiological factors favorable to virus multiplication or translocation are more likely factors