The incidence of rosette disease in groundnut in relation to plant density and its effect on yield

Trials are described in which the relationships between plant density, rosette disease incidence and the yield and quality of groundnuts were investigated on sprayed and unsprayed groundnut plants. During growth, the number and percentage of sprayed and unsprayed plants showing symptoms of rosette disease was significantly greater at low plant populations than at high. At harvest, in general on unsprayed plots the numbers attacked were also greater at low plant populations, but on the sprayed plots the results were inconsistent. The percentage attack at harvest was much lower on sprayed plots at all plant populations. Menazon used as a seed dressing did not affect the incidence of the disease but used as a spray it significantly reduced the amount of disease found. The spray was applied at a rate of 294 g a.i./ha, four times at 10-day intervals, the first spray being applied 10 days from germination. Spraying controlled secondary spread of the virus within the crop and appeared to reduce the severity of early attack. There was a marked improvement of yields and quality of nuts from sprayed plots. Plant density did not affect quality of the groundnuts obtained, but yields per ha were highest on both sprayed and unsprayed plots at the highest plant densities (seed rate up to 134 kg/ha).     

Sub-lethal effects of deltamethrin residues on the within-crop behaviour and distribution ofCoccinella septempunctata

The behaviour and distribution of adultCoccinella septempunctata L. (Coleoptera: Coccinellidae) were recorded in two plots of winter wheat infested with the cereal aphidsSitobion avenae (F.) andMetopolophium dirhodum (Walk.) (Homoptera: Aphididae). One plot was sprayed with the pyrethroid insecticide deltamethrin at a rate of 6.25 g a.i./ha and the other was left unsprayed. Single ladybird beetles were released sequentially on the ground at the centre of the sprayed and unsprayed plots and their behaviour and position in the crop canopy were recorded at 30 second intervals for a total of 15 min per beetle. Assessments, with fresh beetles, continued for four days after spray application with a total of eighty ladybird beetles observed. The 15 min period was selected to avoid lethal effects and no ladybird beetles were killed or knocked down as a result of exposure to deltamethrin residues during this period. Significant differences were found between the overall behaviour patterns ofC. septempunctata in the untreated and deltamethrin treated plots up to three days after the spray application. Ladybird beetles exposed to deltamethrin residues were observed to walk and groom significantly more frequently and to rest significantly less frequently than those in the unsprayed plot. Significant differences were also found between the observed distribution of ladybird beetles in the sprayed and unsprayed crop canopies, with higher numbers of observations towards the bottom of the crop canopy and on the ground in the deltamethrin treated plot than in the untreated plot during the first two days after deltamethrin application. Upon the foliage itself, ladybird beetles were observed significantly more frequently on the abaxial leaf surface in the deltamethrin treated crop compared with the untreated crop. The results are discussed in terms of possible evidence for the repellency of deltamethrin toC. septempunctata and also the implications for integrated pest management of changes in predator behaviour and crop distribution resulting from sub-lethal uptake of insecticides.     

Some factors influencing the performance of spray programmes for the control of coffee berry disease

In 1968 in a field trial at Kamundu, Kenya, early-season (Jan.-Mar.) sprays of copper, captafol and Tuzet failed to control coffee berry disease (CBD), in the late (main) crop of cultivars S.L.28 and S.L.34. Copper and captafol caused some delay in disease development but eventually infection reached over 50 % in all treatments and final yields of sprayed plots were below those from unsprayed plots. In a similar trial in 1969 at Yara on the cultivar French Mission a similar spray programme achieved significant benefits: copper and captafol kept the level of disease below 20% and gave improved yields compared with unsprayed plots. A continuation of the sprays until June increased yields still further. In a second trial in 1968 at Kiamara on the cultivar French Mission substantial quantities of early crop were present and disease in this was effectively controlled by early-season sprays of captafol, although the late crop on the same trees became severely affected. Removal of this overlapping early crop in January caused only a limited delay in disease development in the late crop, despite the fact that with some treatments this stripping resulted in the removal of a large source of potential inoculum from the tree. The results are considered in relation to differences between seasons and fungicides and it is concluded that further evidence has been provided for the importance of the protective role of fungicides against CBD. It is considered that stripping of the early crop is never likely to be a necessary management practice.     

The effects of the foliar fungicide pyrazophos on beneficial arthropods in barley fields

The insecticidal properties of the foliar fungicide pyrazophos were investigated in autumn- and spring-sown barley crops using several methods to estimate the density of non-target arthropods and by using manipulative experiments. In autumn-sown crops, pyrazophos significantly reduced densities of cereal aphid natural enemies relative to pre-treatment levels compared to areas of crop that were left unsprayed. These significant differences persisted in most groups for over 45 days after treatment. Groups most affected were aphid-specific predators, polyphagous predators, (Carabidae, Staphylinidae but not Araneae) and parasitoids. Insects known to be preferred food items of wild gamebird chicks and other non-target groups such as the Collembola were also significantly reduced in density on plots sprayed with pyrazophos, the effects again persisting for over 45 days. Densities of beneficial and non-target insects were also reduced in plots of spring barley sprayed with pyrazophos. However, the experimental design meant that levels of reductions were not significant for most arthropod groups. Removal of prey items (Drosophila pupae) placed in sprayed and unsprayed spring barley plots showed that the proportion predated was significantly lower on pyrazophos-treated plots compared to those remaining unsprayed, the difference persisting for at least 7 wk after treatment. Removal of cereal aphids from colonies introduced to sprayed and unsprayed plots was also significantly lower on plots treated with pyrazophos. Difficulties of data interpretation associated with experimental design are discussed together with the consequences of the use of broad-spectrum pesticides in I.P.M. programmes in cereals.     

Yield loss of faba bean (Vicia faba) due to chocolate spot (Botrytis fabae) in sole and mixed cropping systems in Ethiopia

Two field experiments were conducted during 2004 and 2005 cropping seasons at Adet Agricultural Research Center, Ethiopia to assess yield losses caused by chocolate spot(Botrytis fabae) of faba bean in sole and mixed cropping systems using two cultivars. Cropping systems were sole faba bean (FB), faba bean mixed with field pea (FB: FP), barley (FB: BA) and maize (FB: MA). Mancozeb was sprayed at the rate of 2.5 kg a.i/ha at 7-, 14- and 21-day interval to generate different levels of chocolate spot disease in all the four cropping systems, and unsprayed control was also included. The treatments were arranged in a randomised complete block design (RCBD) with four replications. FB: MA mixed cropping significantly reduced disease severity and the area under disease progress curve (AUDPC) and increased faba bean grain yield. The highest faba bean grain yield among the three mixed croppings under different spray schedules was obtained from FB: MA mixed cropping in both 2004 and 2005 (2.56 and 3.74 t/ha, respectively) cropping seasons. There were highly significant yield differences (P < 0.05) among the spray intervals of mancozeb in both seasons. The highest grain yield (4.9 t/h) was recorded from the 7-day spray interval in 2005. The unsprayed faba bean had a lower grain yield (1.9 t/ha in 2004 and 2.3 t/ha in 2005) compared to the sprayed plots. The highest relative yield loss (67.5%) was calculated in 2005 from FB: FP mixed cropping in unsprayed plots. The relative yield losses in the unsprayed plots were in the range of 35.8–41.5% in 2004 and 52.6–67.5% in the 2005 cropping season. Severity and AUDPC were inversely correlated with faba bean grain yield. Significant differences were recorded in the 100-seed weight and days to maturity (DM). The unsprayed plots had shorter DM ranging from 126 to 128.5 day (except FB: MA mixed cropping) in 2004 and 122–123.9 days in 2005. In the sprayed plots DM was relatively longer than the unsprayed plots. A higher seed weight was recorded in the sole FB (56 g) and FB: MA (55 g) mixed cropping, and the lowest value of 100-seed weight was recorded from FB: FP (53 g) mixed cropping. The productivity of the mixed cropping evaluated by land equivalent ratio (LER) exceeded that of sole cropping. Faba bean grain yield was highly influenced by the severity of chocolate spot. The disease affects the DM, forcing early maturing of the plants.     

The survival of gamebird (Galliformes) chicks in relation to pesticide use on cereals

Field experiments were carried out to test the effects of cereal pesticides (herbicides, fungicides and insecticides) on chick survival of Grey Partridge Perdix perdix , Red-legged Partridge Alectoris rufa and Pheasant Phasianus colchicus. On fields in experimental plots the outer 6 m of cereal (the headland) were not sprayed with pesticides from 1 January 1984, whereas control plots were fully sprayed. Gamebird brood counts were carried out after the cereal harvest. In addition, nine Grey Partridge broods were radio-tracked for 21 days after hatching (four in sprayed plots and five in unsprayed plots) to determine their movements, home range size and survival in relation to pesticide spraying.
The mean brood size of Grey Partridge and Pheasant was significantly higher on plots where field edges were unsprayed than on fully sprayed control plots. Data for Red-legged Partridge were inconclusive. The survival of individually marked Grey Partridge broods was negatively related to the distance moved between successive nocturnal roost sites. Survival was significantly higher, the distance moved between roost sites significantly shorter and the proportion of home range including headland significantly greater for broods feeding in spring barley fields with unsprayed field edges compared with broods feeding in fully sprayed fields.     

Control of coffee berry disease*

Field experiments comparing six spraying programmes and two fungicides for control of coffee berry disease were carried out at three sites during 1968. These established that protection of the crop by sprays applied throughout the long rains (February–July) gave the best control and increased yields threefold. Early season spray programmes, which reduced the sporulating capacity of the pathogen in coffee twigs, but which ceased before the end of the rainy season, gave no disease control in the late crop (harvested October-December) and yields were less than in unsprayed controls. All programmes which gave protection at the onset of the rains gave some disease control in the early crop (harvested June-August). Captafol (‘Ortho-Difolatan’ 80 WP) was better than a 50% copper formulation (‘Perenox’). Conditions suitable for infection, as determined from meteorological records, occurred frequently during both rainy seasons. Estimates of sporulating capacity and data from spore trappings showed that although inoculum from the bark might initiate an epidemic, subsequent disease progress was more dependent upon spores derived from diseased berries; these accounted for most of the inoculum dispersed during the greater part of the season. Removal of the early crop, which was very small, had no detectable effect on disease in the late crop. Although losses were greatest with early season spray programmes, these did nevertheless delay the onset of the epidemic. However, the subsequent rate of disease increase was greatest and ‘scab’ lesion formation least in these treatments. ‘Scab’ lesions were most abundant in those treatments which caused least delay; in unsprayed coffee, it appeared that early exposure to infection resulted in ‘scab’ lesion formation which subsequently retarded the rate of disease progress. Tree height was positively correlated with disease incidence, but there was little effect of tree aspect. Crop density had a small effect on disease incidence within branches.     

Population fluctuations of the diamondback moth,Plutella xylostella (L.) on cabbages inBacillus thuringiensis sprayed and non sprayed plots and factors affecting within-generation survival of immatures

Summary The population fluctuations and within-generation survival of immatures stages of the diamondback moth,Plutella xylostella on summer cabbages, were examined in unsprayed plots in 1984 and 1985 and in plots sprayed with two formulations ofBacillus thuringiensis in 1985. There was two distincy generations per crop and no noticeable difference in population fluctuations were observed in unsprayed plots between years. There was distinct difference between unsprayed and sprayed plots in 1985, from the larval period onwards. However, the survivorship patterns in all plots in both years were a Type 2 based onDeevey's (1947) classification. Life table studies showed that there was essentially no difference in the mortality agents acting on each of the stages except for numerical differences in the within-generation mortality rates. The major mortality rates during the egg stage were the parasitoid,Trichogramma spp. and unknown factors including rainfall; in the larval 1 stage was unknown mortality; in the larval 2 stage was the parasitoid,Apanteles plutellae and during the pupal stage was unknown mortality and parasitoid,Diadromus subtilicornis. Adult mortality was determined for generation 2. It was relatively higher in the unsprayed plots compared to theB. thuringiensis sprayed plots. The contribution of abiotic factors such as rainfall and temperature, and biotic factors such as parasitoids and predators in determining within-generation population levels and the fluctuation of populations on cabbage were discussed.     

Accumulation and persistence of a nuclear polyhedrosis virus of the cabbage looper in the field

A nuclear polyhedrosis virus of the cabbage looper, Trichoplusia ni, was applied to foliage of Fall-planted broccoli and cabbage plants. Soil bioassays demonstrated that appreciable quantities of active virus accumulated in the upper 1 cm of the soil of plots sprayed with 10, 20, and 100 larval equivalents (LE)/acre as well as in unsprayed plots. After completion of the spray program, soil from plots treated with 8 foliar applications of 10 LE/acre contained only slightly more virus than the soil from unsprayed plots, but considerably more virus accumulated in plots treated with 20 and 100 LE/acre. The accumulation of virus continued after the completion of spraying and considerable quantities of active virus persisted through the winter.     

The effect of sprays of thiophanate–methyl on cane diseases and yield in red raspberry, with particular reference to cane blight (Leptosphaeria coniothyrium)

Spur blight (Didymella applanata), cane botrytis (Botrytis cinerea) and cane blight (Leptosphaeria coniothyrium) were studied in two consecutive seasons in unsprayed raspberries and in plots sprayed with thiophanate-methyl in the first season alone, (a) twice pre-harvest, (b) twice post-harvest, or (c) twice pre- plus twice post-harvest. Pre-harvest, but not post-harvest sprays, gave moderate control of both spur blight and cane botrytis; both diseases occurred mainly on the lower halves of canes, the latter being the less common. Cane blight occurred at the base of canes where they were wounded by old cane stubs. In the first year it was severe; 37% of canes died before harvest in the unsprayed plots. The three spray programmes all decreased cane death due to cane blight and they all increased yield by c. 45% even in those plots sprayed post-harvest where spur blight was severe and not controlled. Clearly this last disease, despite its high incidence, had no effect on potential yield in this experiment. In the second year cane blight was common but less severe and the incidence of its lesions was reduced similarly by all programmes, but only 4% of canes died in control plots. Analysis of the potential yield and the sizes of lesions caused by L. coniothyrium in canes inoculated at fortnightly intervals in the previous year showed that potential yield loss occurred only when lesions girdled canes. A bimodal distribution in the lesion sizes, measured by length or girdling, indicated some endogenous control of lesion development which might explain the marked differences in the effect of cane blight on potential yield in two seasons.     

Use of potassium bicarbonate (Armicarb) on the control of powdery mildew (Sphaerotheca mors-uvae) of gooseberry (Ribes uva-crispa)

Powdery mildew (Sphaerotheca mors-uvae) severely infects young shoots, stems and fruits of gooseberry (Ribes uva-crispa). Environmental friendly and biological control measures are being sought throughout the world. Especially in organic gooseberry growing effective control measures are needed, because powdery mildew infections may result in a total loss of the crop. In organic currant growing the number of adequate control methods is very limited. Sulphur as a fungicide against powdery mildew in e.g. gooseberry or table grape growing is not recommendable due to possible bleaching of berries and scorching of tender shoots. Various bicarbonate salts are suggested as a good option to control powdery mildew. In a field trial the effect of potassium bicarbonate (Armicarb) on the on the control of powdery mildew of gooseberry was evaluated. Four treatments; i.e. two preventive strategies and two curative strategies, were applied. PLants were sprayed until runoff. The percent of infected fruits and disease severity were assessed. In the unsprayed control plots very high disease incidences were observed; on average more than 90% of the berries were infected with powdery mildew. The weekly (preventive) spray applications with potassium carbonate reduced the disease incidences on fruits significantly. On average approximately 10% of the fruits were affected by powdery mildew. However, the number of spray applications was high. In conclusion, our results indicate that applications of potassium bicarbonate (as Armicarb) are effective in reducing the incidence and severity of American powdery mildew in gooseberry. Early spray applications are necessary to protect berries against powdery mildew infections. Future research will focus on reducing the number of applications, e.g. warning models based on powdery mildew of rose (Sphaerotheca pannosa).     

Interactions of crop density of field beans, abundance of Aphis fabae Scop., virus incidence and aphid control by chemicals

The number of Aphis fabae Scop. per plant and per acre developing on field beans (Vicia faba L.) was inversely related to seeding rate (i.e. plant density) except sometimes at very low rates; with equal numbers of plants per acre, fewer aphids developed on plants in rows 11 in. than 22 in. apart. Plots sown in mid-March with more than about 150,000 plants per acre were more attractive than less dense stands to colonizing alate A. fabae, but established colonies multiplied most on the sparsest and least on the densest plots. The number of plants per acre infected by pea leaf-roll virus was inversely related to planting density. There were more virus-infected plants on II in. than on 22 in. spaced rows-in contrast to the numbers of A. fabae. A single spray with demeton-methyl, timed to control A. fabae, did not significantly decrease virus incidence. Grain yields of sprayed plots were little altered by increasing the seed rate above a critical minimum, except in one year when the densest crops lodged. Increased yields from spraying were closely related to the numbers of A. fabae on unsprayed plots. Dense planting (more than 400,000 plants per acre) prevented or greatly decreased losses caused by A. fabae in unsprayed plots except in one year when the aphids were exceptionally abundant.     

Effects of fungicides on the populations of Colletotrichum and other fungi in bark of coffee*

The effects of copper, captafol and Tuzet, applied to coffee in early season (January to April) and in other programmes continuing through the long rains (January to July), on the bark populations of Colletotrichum cofleanum Noack. (the pathogenic CBD strain and the saprophytic ccm, cca and ccp strains), Phoma spp., Fusarium stilboides Wollenw. and a Phomopsis sp. were investigated on branch material from field experiments in Kenya in 1968 and 1969 by a plating technique which estimated sporulating capacity (s.c.) Fungicide application initially caused a non-specific reduction in s.c., Tuzet being the most effective. With the onset of the long rains this general reduction was not maintained, although during the period between the long and short rains the levels of many fungi in sprayed plots were still below those in the unsprayed control. In contrast, the s.c. of Phoma in plots sprayed with copper and of Phomopsis in those sprayed with Tuzet were higher than on unsprayed plots. The S.C. of the CBD strain was below that in the control only in plots sprayed with captafol. Effects were still evident after the short rains. Some early-season programmes of all three fungicides increased the s.c. of the CBD strain while continuing programmes of copper and captafol reduced its S.C. relative to that on unsprayed trees. Differences in S.C. at this time were related to infection in the early crop a few weeks later. The initial antisporulent effect of a fungicide appears less important than its persistence and its effect on bark colonization by the different fungi. Early-season sprays of copper, formerly used to control CBD, may have contributed to an increase in disease hazards by affecting the primary level of the pathogen.     

Effectiveness of GF-120 fruit fly bait spray applied to border area plants for control of melon flies (Diptera: Tephritidae)

In a field study in Hawaii, color-marked protein-deprived and protein-fed female melon flies, Bactrocera cucurbitae Coquillett, were released within canopies of unsprayed sorghum plants (a nonhost of melon flies) outside of a border area of unsprayed or bait-sprayed sorghum plants or open space that surrounded cucumbers, a favored host of melon flies. Application of bait spray to sorghum or sugarcane surrounding host plants of melon flies is a common practice for melon fly control in Hawaii. GF-120 Fruit Fly Bait spray proved very effective in preventing protein-deprived females from alighting on cucumbers (23% of released females were observed dead on bait-sprayed sorghum; 0% were observed alive on cucumbers), but proved less effective in suppressing protein-fed females (14% of released females were observed dead on bait-sprayed sorghum; 11% were observed alive on cucumbers). No females were found dead on unsprayed sorghum. Compared with open space surrounding cucumbers, the presence of unsprayed sorghum as surrounding border area neither significantly enhanced nor significantly inhibited the ability of either type of female with respect to finding cucumbers. Greenhouse cage assays revealed that compared with droplets of water, droplets of GF-120 Fruit Fly Bait spray were highly attractive to protein-deprived females within 1 h of bait spray application to sorghum, but lost about half of their attractiveness within 5 h and all of it within 24 h under the dry greenhouse conditions used for maintaining baited-sprayed sorghum plants in these assays. Laboratory cup assays showed that bait spray droplets remained highly toxic to protein-deprived females 24 h after application, but lost nearly half of their toxicity within 4 d under laboratory exposure and nearly all of it after approximately 8 mm of rainfall. Combined findings suggest that application of GF-120 Fruit Fly Bait spray to nonhost plants for melon fly control either be made often enough to overcome loss of attractiveness of bait spray droplets to females or that bait spray be applied to nonhost plants that are themselves attractive to the females.     

Field experiments on the chemical control of blackcurrant reversion virus and its gall-mite vector (Phytoptus ribis Nal.)

Healthy blackcurrant bushes and others infected with reversion virus were exposed equally to infestation by the gall-mite vector (Phytoptus ribis Nal.) spreading from unsprayed sources planted nearby. Four applications of 1·0% lime sulphur at fortnightly intervals during the dispersal period were less effective than 0·05% endosulfan in preventing the infestation of healthy bushes, whereas endrin at 0·04% gave almost complete control. All spray treatments were much less effective when applied to the virus-infected bushes, although endosulfan remained superior to lime sulphur and inferior to endrin. Infected bushes sprayed with lime sulphur developed more galls than unsprayed healthy bushes. In another experiment, four applications of endrin or lime sulphur decreased the spread of mites and reversion virus from infested bushes to adjacent sprayed and unsprayed bushes. Lime sulphur was much less effective than endrin in preventing infestation with mites, but more effective in decreasing the incidence of virus. These results are discussed in relation to commercial spray programmes and to the design of experiments on the effectiveness and mode of action of spray materials against mites and virus.     

Potential of pigeon pea as a trap crop for control of fruit worm infestation and damage to okra

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Beauveria bassiana for the control of Sunn Pest (Eurygaster integriceps) (Hemiptera: Scutelleridae) and aspects of the insect's daily activity relevant to a mycoinsecticide

A series of investigations was carried out at ICARDA during April-June 2004 and May-June 2005 to investigate the use of the entomopathogenic fungus Beauveria bassiana to control Eurygaster integriceps and to determine aspects of the insect's biology that could be relevant to control with the fungus. Application in 2004 of an oil-based formulation of B. bassiana showed distribution of the spray decreasing from the top of the wheat to the bottom, although the proportion of tillers with coverage at the base was still 80%. Seventy percent of Sunn Pest had received direct hits, with the highest amount on the dorsal surface, followed by the ventral surface, then the legs and antennae. Mortality of field-treated, then laboratory-maintained insects, ranged from 47.6% for insects exposed only to the B. bassiana spray, 74.2% for unsprayed insects exposed to treated wheat and 86.4% for insects receiving direct spray and being exposed to treated wheat. Some field results demonstrated mortality. Mortality assessments following the 2005 treatments were conducted solely in the field, there was no apparent effect upon Sunn Pest numbers. The majority of studies on Sunn Pest biology were carried out on untreated insects outside the trial plots. Different assessment methods resulted in varying estimates of adults and juveniles in the field. The numbers of Sunn Pest found and the proportion of these at the top of the wheat crop, declined after early morning and increased again in the early evening, suggesting sheltering deeper in the crop or in the soil during the middle part of the day. Mobility and location of marked insects was followed in micro-plots and by mark and recapture studies, demonstrating a range of activities, from movement around the crop for a number of metres, including contact with numerous tillers to almost total immobility; Sunn Pest generally avoided movement on the soil. The potential for Sunn Pest control with a mycoinsecticide and how Sunn Pest behaviour is relevant to a mycoinsecticide application are discussed.     

Invasion of a West Everglades wetland by Melaleuca quinquenervia countered by classical biological control

The population dynamics of Melaleuca quinquenervia were monitored over a 5-year period in a cypress-pine wetland while subjected to two levels of herbivory. The trees had been recruited during 1998–1999 after a destructive crown fire. Half of 26 experimental plots were sprayed every 4–6 weeks with an insecticide to reduce herbivory by the biological control agents Oxyops vitiosa and Boreioglycaspis melaleucae. After only 1-year melaleuca density increased by 26% in sprayed plots and by 7% in unsprayed plots. However, over the entire 5-year period melaleuca density increased in sprayed plots by 0.1% while decreasing by 47.9% in unsprayed plots when compared to initial densities. Annual mortality of melaleuca never exceeded 6% in any year in sprayed plots but ranged from 11% to 25% in unsprayed plots. There was a significant year by treatment interaction indicating the importance of the environment on tree mortality. Limited seed production occurred on sprayed trees but never on unsprayed trees. Mean tree height increased by 19.6% in sprayed plots while declining by 30.6% in unsprayed plots. Coverage by native vegetation did not increase with decreasing melaleuca density. This is the first study with controls that quantifies the population level regulation of melaleuca by introduced biological control agents and corroborates other correlative studies that documented significant changes in melaleuca communities after the introduction and establishment of biological control agents.     

The effect on grain yield of adult plant resistance to mildew in oats

Lines were selected from progeny of the crosses Maldwyn × Milford and Maldwyn × Sun II which consistently exhibited small and large amounts of mildew in similar situations. These are referred to as ‘low’ and ‘high’ mildew lines respectively. In small plot field trials conducted in 1971, 1972 and 1973 the low mildew lines produced substantially more grain each year compared with the high mildew lines in plots where mildew was allowed to develop normally. In plots which were kept comparatively free from infection by spraying with tridemorph, both low and high mildew lines gave similar yield. When the unsprayed plots were compared with the sprayed plots the low mildew lines invariably showed very small losses in grain yield and the high mildew lines usually very large losses. This same trend was evident also in the parent cultivars in 1972. Maldwyn, with only 7·9% mildew cover in the unsprayed plots due to its resistance, suffered a decrease of only 9·2% in grain yield compared with losses of 17·1 and 20·3% in the more susceptible cultivars Milford and Sun II. Certain lines and cultivars sustained heavy levels of mildew without corresponding loss of grain yield, i.e. showed low sensitivity to the disease. The importance of combining low sensitivity (or tolerance) with adult plant resistance to mildew is discussed.     

The effects of sowing date and choice of insecticide on cereal aphids and barley yellow dwarf virus epidemiology in northern England

During the mid-1980s, Sitobion avenae became recognised as an important vector of barley yellow dwarf virus (BYDV) in the Vale of York. A field trial at the University of Leeds Farm, North Yorkshire, was carried out during the autumn/winter of 1984-85 to evaluate different control procedures against S. avenae-transmitted BYDV and to investigate its epidemiology. Winter barley was sown on three dates in September, and plots were sprayed with either deltamethrin, demeton-S-methyl or pirimicarb on one of three dates between mid-October and mid-November, making a factorial design. Rhopalosiphum padi, the main vector of BYDV in southern England, were rarely found during the experiment, but the numbers of S. avenae were much higher, reaching a peak of 21% of plants infested in the unsprayed plots of the first sowing date. Single applications of each insecticide reduced populations of S. avenae to zero. Some treatments, particularly in the early sown plots and those treated with pirimicarb, however, did allow some recolonisation, and thus led to increased virus incidence and decreased yields. Sprays applied before the end of the migration of S. avenae were more efficient at controlling BYDV if the insecticide was persistent, otherwise a spray after this period, in November, was more effective. Virus incidence, although reduced by sprays, was generally low in plots sown on 18 and 27 September. In contrast, about 11% of plants were infected in unsprayed plots sown on 6 September and a small yield benefit was obtained with insecticidal treatments. Enzyme-linked immunosorbent assay (ELISA) of plants taken from the plots indicated that MAV- and PAV-like strains were present, and were most likely to have been transmitted by S. avenae.