Virus infection stimulates phyto-oestrogen production in pasture legume plants growing in grazed swards

Four field experiments were sown with AMV‐infected or healthy seed of burr medic (Medicago polymorpha) and grazed by sheep; two were sown with cv. Circle Valley and two with cv. Santiago. Seed‐infected plants acted as primary sources for virus spread by naturally occurring aphids. Insecticides and admixture with annual ryegrass (Lolium rigidum), a non‐host of AMV, suppressed virus spread to different extents in the plots sown with infected seed. Effects of the different amounts of virus spread obtained on overall concentration of the oestrogenic compound coumestrol (dihydroxycoumestan) in dry stems and pods, and on seed production were measured in the medic. With cv. Santiago, stem and pod coumestrol concentration values for plots sown with healthy seed were significantly smaller than those for all plots sown with infected seed regardless of whether they were sprayed. With cv. Circle Valley, the coumestrol values for stems from plots sown with healthy seed were significantly smaller than those for unsprayed plots sown with infected seed but not than those for sprayed plots or ones with grass admixtures, and there were no significant differences with pods. There was always a significant positive relationship between concentration of coumestrol in medic stems and percentage AMV infection of swards; this was also so with pods in two experiments. A linear model best fitted this relationship with cv. Circle Valley but a logarithmic model did so with cv Santiago. In glasshouse grown plants, the coumestrol content of dried medic shoots was increased 11 (cv. Circle Valley) and five (cv. Santiago) times by AMV infection. AMV increased mean coumestrol concentrations up to 256 ppm (field) and 237 ppm (glasshouse) in stems and 223 ppm in pods (field). Sowing healthy seed in new pasture swards was an effective strategy for minimising coumestrol accumulation in burr medic swards. Two, but not single, applications of a newer generation pyrethroid insecticide to swards in which AMV was spreading significantly diminished coumestrol accumulation but applying organophosphorus insecticide twice and carbamate insecticide repeatedly did not. Medic seed yields and individual seed weights were sometimes significantly increased by the treatments, suppression of AMV spread by regular carbamate sprays being sufficient to increase seed yield by 55%.     

Alfalfa mosaic and cucumber mosaic virus infection in chickpea and lentil: incidence and seed transmission

Samples collected in 1994 and 1995 from commercial crops of chickpeas and lentils growing in the agricultural region of south-west Western Australia were tested for infection with alfalfa mosaic (AMV) and cucumber mosaic (CMV) viruses, and for members of the family Potyviridae using enzyme-linked immunosorbent assay (ELISA). In 1994 no virus was detected in the 21 chickpea crops tested but in 1995, out of 42 crops, AMV was found in two and CMV in seven. With lentils, AMV and/or CMV was found in three out of 14 crops in 1994 and 4 out of 13 in 1995, both viruses being detected in two crops in each year. Similar tests on samples from chickpea and lentil crops and plots growing at experimental sites, revealed more frequent infection with both viruses. No potyvirus infection was found in chickpeas or lentils in agricultural areas either in commercial crops or at experimental sites. However, bean yellow mosaic virus (BYMV) was detected along with AMV and CMV in irrigated plots of chickpeas and lentils at a site in Perth. When samples of seed from infected crops or plots of chickpeas and lentils were germinated and leaves or roots of seedlings tested for virus infection by ELISA, AMV and CMV were found to be seed-borne in both while BYMV was seed-borne in lentils. The rates of transmission found through seed of chickpea to seedlings were 0.1–1% with AMV and 0.1–2% with CMV. Seed transmission rates with lentil were 0.1–5% for AMV, 0.1–1% for CMV and 0.8% for BYMV. Individual seed samples of lentil and chickpea sometimes contained both AMV and CMV. With both species, infection with AMV and CMV was sometimes found in commercial seed stocks or seed stocks from multiplication crops of advanced selections nearing release as new cultivars. Seed-borne virus infection has important practical implications, as virus sources can be re-introduced every year to chickpea and lentil crops or plots through sowing infected seed stocks leading to spread of infection by aphid vectors, losses in grain yield and further contamination of seed stocks.     

Seed-borne cucumber mosaic virus infection of subterranean clover in Western Australia

In Western Australia, infection with cucumber mosaic virus (CMV) was widespread in all three subspecies of subterranean clover (Trifolium subterraneum) growing in plots belonging to the Australian National Subterranean Clover Improvement Programme. Seed-borne CMV was detected in seed harvested in 1984–1986 of 18/25 cultivars from two collections of registered cultivars; seed transmission rates ranged up to 8.8%. Seed samples from CMV-inoculated plants of 11 cultivars transmitted the virus to 0.5–8.7% of seedlings. Seed transmission rates greater than 5% were obtained only with cvs Enfield, Green Range and Nangeela. CMV was not detected in seed harvested in 1975–1981 from one of the registered cultivar collections, in 17 commercial seed stocks from 1986 or in a survey of subterranean clover pastures.
Symptoms in subterranean clover naturally infected with CMV included mottle, leaflet downcurling and dwarfing but severity varied with cultivar and selection. CMV isolates from different sources varied in virulence when inoculated to subterranean clover; two (both from subterranean clover) were severe, two moderate and three (including one from subterranean clover) mild. In pot tests, CMV decreased herbage production and root growth (dry wts) of cv. Green Range by 49% and 59% respectively. In spaced-plants growing in plots, CMV decreased herbage production and root growth of cvs Green Range and Northam by 59–630 and seed production of cv. Green Range by 45%. In rows sown with infected seed, aphid spread increased infection levels to 75% in cv. Green Range and 44% in cv. Esperance and losses in herbage production of 42% and 29% respectively were recorded.
CMV isolated from subterranean clover included isolates from both serogroups.     

Losses in productivity of subterranean clover swards caused by sowing cucumber mosaic virus-infected seed

Field trials were done in 1988 - 89 at two sites to examine the effects of sowing seed stocks in which a low proportion (1.6–7.0%) of the seed was carrying cucumber mosaic virus (CMV) infection (= infected seed) and the subsequent CM V spread that results, on the productivity of swards of subterranean clover cvs Esperance, Green Range and Karridale. Except in irrigated plots of cv. Green Range, a variable proportion of the CMV-infected seedlings always failed to establish, so sowing infected seed normally resulted in plots containing fewer or far fewer seed-infected plants than expected. The rate of virus spread by aphids was faster and resulted in more extensive infection at maturity when the plots contained more seed-infected source plants. In two irrigated trials at South Perth, in which healthy and infected seed of cvs Esperance and Green Range was sown, CMV spread was extensive. When the plots were left undefoliated, herbage dry wt yields were decreased by 12 – 30% and seed yields by 53 – 64% due to infection. When they were mown, the herbage dry wt losses recorded were 17 – 24%. In three trials at Mt Barker sown with healthy and infected seed, extensive spread of CMV occurred with cv. Green Range but not with cvs Esperance and Karridale. With cv. Green Range, losses of 25 – 28% in herbage dry wt were recorded inside CMV-affected patches in mown or grazed plots, while losses were up to 13% when herbage was sampled at random. Seed yield losses were 40–42% and 53% in infected mown and undefoliated cv. Green Range plots, respectively. In the mown or grazed plots of cvs Esperance and Karridale, herbage dry wt losses recorded were up to 7% while seed yield losses were 9 – 16% in mown and 9% in undefoliated plots. The mean wt/seed of seed harvested from mown plots of cvs Green Range and Karridale sown with infected seed was 8–12% less than that of seed from mown control plots. CMV was detected in seed harvested from undefoliated cv. Green Range plots and mown plots of cvs Green Range and Karridale sown with infected seed but levels of seed infection with the mown plots were 3–5 times less than in the seed sown. Field trials were done at two sites in 1987 – 90 to examine the persistence of CMV in subterranean clover swards. CMV infection was established in 1987 and the plots were grazed in subsequent years. At Badgingarra, infection gradually decreased with little CMV being recovered by 1990. At Mt Barker, recovery of CMV was relatively poor in 1988 and even poorer in 1989, but there was some resurgence of CMV infection in 1990.     

Increasing phosphorus concentration in seed of annual pasture legume species increases herbage and seed yields

In glasshouse experiments with low levels of soil applied phosphorus (P), yields of four annual pasture legumes (Medicago polymorpha, Trifolium subterraneum, T. balansae, Ornithopus compressus) increased with increasing P concentration in the seed. In a further experiment, M. polymorpha cv. Serena was grown at the same plant density from seed of two P concentrations and two seed sizes when two levels of finely ground superphosphate were applied to the soil. Higher P concentrations in the seed increased yields of dried tops by about 30% for the first harvest (21 days), 20% for the second harvest (52 days), and 9% at maturity (103 days), and seed yields by 11%. Larger seeds increased yields of dried tops by between 6–46% for the first two harvests but at maturity yields of dried tops and seed were unaffected by seed size. None of the interactions were statistically significant (P>0.05), except for the first harvest when two interactions (P concentration in the seed × seed size (i.e. P content in seed), and P applied to the soil × P concentration in the seed × seed size) were significant at P<0.05 level. In a field experiment, Trifolium subterraneum clover seed (two cultivars) of the same size but with two different P concentrations was sown at the same plant density and two levels of granulated (0.2–5 mm) superphosphate were applied to the soil surface. The higher level of superphosphate increased dried herbage yields of the dense clover swards by three- to four-fold 90 and 120 days after sowing. The higher P concentration in the seed increased yields of dried herbage by between 50 to 25%, depending on the level of P applied to the soil and the harvest date.     

Seed-borne cucumber mosaic virus infection of narrow-leafed lupin (Lupinus angustifolius) in Western Australia

In 1986 in Western Australia, cucumber mosaic virus (CMV) infection was widespread in breeders' selections of narrow-leafed lupin (Lupinus angustifolius), and in collections of lupin cvs and wild L. angustifolius lines. When seed of some of these selections and cvs was sown, seed-borne CMV was detected in seedlings. Infection of F₁ progenies was traced to use of infected parent plants. CMV was also widespread in 25 seed crops of the new lupin cv. Wandoo but not in 42 seed crops of the new cv. Danja. When samples of the seed sown in 1986 were tested, CMV was detected in 3 - 34% of seedlings of cv. Wandoo but in none of cv. Danja. Following intensive roguing of symptom-bearing plants in the 1986 seed crop of new lupin cv. Gungurru, the level of seedling infection with CMV in seed samples after harvest was 0·1-0·2%. CMV was detected in 6 - 8%, 0·6-5% and 0 - 18% of seedlings from seed samples of established lupin cvs Chittick, Yandee and Illyarrie respectively. Highest levels of seed transmission were in seed from crops grown in high rainfall areas. When a sample of cv. Wandoo seed was graded for size by sieving, CMV was detected in seedlings grown from seed in all grades, but the smallest grade contained the highest level of infection. When seed was collected from pods at different positions on plants in a CMV-infected crop of cv. Illyarrie, seed from primary pods transmitted the virus to seedlings at a 3% rate, seed from first order lateral pods at 8% while seed from second and third order lateral pods transmitted at 13%. Examination of CMV-infected lupin crops indicated that seed-infected plants competed poorly and tended to be shaded out in dense crops but to survive in sparse crops. In 1987 during drought conditions after seeding, plant mortality was greater with seed-infected seedlings than with healthy seedlings despite wide plant spacing. An isolate of CMV from subterranean clover (Trifolium subterraneum) induced severer symptoms in lupins than four isolates from lupin; only the subterranean clover isolate prevented seed production. In tests at one lupin breeding site, CMV was found in 15 species of weeds and volunteer legumes. Fumaria officinalis, Stachys arvensis and volunteer lupins were most frequently infected.     

Suppressing spread of alfalfa mosaic virus in grazed legume pasture swards using insecticides and admixture with grass, and effects of insecticides on numbers of aphids and three other pasture pests

Field experiments were sown with alfalfa mosaic virus (AMV)‐infected or healthy seed of burr medic (Medicago polymorpha) and grazed by sheep. Seed‐infected plants acted as primary sources for virus spread by naturally occurring aphids. Admixture with annual ryegrass (Lolium rigidum), a non‐host of AMV, and different insecticides were used in attempts to suppress virus spread. Sowing swards to provide the ratios 1 : 4 and 1 : 13 of medic:ryegrass plants diminished AMV spread in medic plants by 23% and 45% respectively. Applications of organophosphorus (demeton‐s‐methyl), carbamate (pirimicarb) and newer generation synthetic pyrethroid (alpha‐cypermethrin) insecticides, all significantly decreased final AMV incidence. Alpha‐cypermethrin was the most effective, suppressing AMV incidence by 87% (two sprays), 79% (one late spray) and 65% (one early spray). Two sprays of demeton‐s‐methyl decreased incidence by only 36%, while two and 2 weekly applications of pirimicarb diminished it by 29–65% and 35–70% respectively. AMV infection of medic seed harvested decreased by up to 76% in sprayed plots. Insecticide treatment did not prevent winged aphids from landing but numbers of wingless Acyrthosiphon kondoi colonising swards were suppressed by up to 92% by spraying with pirimicarb and up to 96% by alpha‐cypermethrin. A. kondoi were much slower to recover with alpha‐cypermethrin than with pirimicarb, the former still significantly diminishing its numbers 35 days after spraying. Alpha‐cypermethrin was also very effective at suppressing Halotydeus destructor and Penthaleus major but not Sminthurus viridis. Greater effectiveness of insecticides in controlling spread of AMV in pasture than has been found previously with non‐persistently aphid‐transmitted viruses in annual crops seems due to the key role played by wingless aphids as virus vectors.     

Seed transmission of turnip yellow mosaic virus in winter turnip and winter oilseed rapes

This is the first record of seed transmission of turnip yellow mosaic virus (TYMV) in oilseed and turnip rapes. The seed transmission of TYMV in a naturally infected winter turnip rape (Brassica napus var. silvestris) cultivar Perko PVH was investigated. By ELISA 1.6%, 3.2% and 8.3% seed transmission of the virus was found in seed of plants from three localities. The proportion of infected seeds produced by artificially infected plants of winter oilseed rape (Brassica napus ssp. oleifera) and winter turnip rape cultivars was determined. The virus transmission rate, expressed as the proportion of virus-infected plants which germinated from the seed was for the oilseed rape cvs Jet Neuf 0.1%, Solida 0.4%, Silesia 0.8%, Darmor 1.2%, SL-507 0.2%, SL-509 0.0% and for the winter turnip rape cv. Perko 1.5%. ELISA cannot be used in direct tests on bulk seed lots to estimate proportion of infected seed, but must be used on germinated seedlings.     

Isolates of raspberry bushy dwarf virus differing in Rubus host range

Isolates of raspberry bushy dwarf virus (RBDV) occurring in the field at East Mailing Research Station (EMRS), and an isolate from raspberry seed imported from the USSR, were found to differ from the Scottish type isolate (D200) of RBDV in that they infected red raspberry cultivars that are resistant, possibly immune, to isolate D200. Of several red raspberry, blackberry and hybrid berry cultivars and EMRS raspberry selections graft-inoculated with these recently discovered RBDV isolates only two raspberry cvs (Haida and Rannaya Sladkaya) and one EMRS selection did not become infected. Differences in the conclusions reached in two previous studies on the inheritance of resistance to RBDV in raspberry can be explained by the use of virus isolates that differed in Rubus host range.     

Resistance and susceptibility to colour forms of the aphid Sitobion avenue in spring and winter wheats (Triticum aestivum)

Glasshouse assessments of resistance to S. avenae in 29 entries of wheat and two of rye were made by releasing half-grown aphids on randomised plants at the stem extension phase of growth. Wheat cvs Kador, Amigo, Highbury and Lutescens 1377 were resistant and cvs Sentry and Talavera de Bellevue partially resistant. Cv. Klein Acero, a breeding line TB68/6/10 and Lerma Rojo selections 197 to 200 were highly susceptible to S. avenae although the latter are moderately resistant to greenbug (Schizaphis graminum) (Starks & Merkle, 1977). The rye cultivars were susceptible to 5. avenae and no cultivar was found to be resistant to Metopolophium dirhodum. Clonal stocks of S. avenae, differing in colour, varied in their ability to form large populations on susceptible cultivars, and hence in their differentiation of susceptible from resistant wheat. No clone was detected with specific ability to attack the resistant cvs Kador and Amigo.     

The isolation and identification of rhubarb viruses occurring in Britain

Virus-like symptoms were common in British crops of rhubarb. All plants tested of the three main varieties, ‘Timperley Early’, ‘Prince Albert’ and ‘Victoria’, were virus-infected. Turnip mosaic virus and a severe isolate of arabis mosaic virus (AMV) were obtained from ‘Timperley Early’; and ‘Prince Albert’ contained turnip mosaic virus, cherry leaf roll virus (CLRV), a mild isolate of AMV and, infrequently, cucumber mosaic virus (CMV). The main commercial variety ‘Victoria’ contained turnip mosaic virus, CLRV, a mild isolate of AMV and, infrequently, strawberry latent ringspot virus (SLRV). All the viruses were identified serologically. The rhubarb isolates did not differ markedly from other isolates of these viruses in herbaceous host reactions, properties in vitro or particle size and shape. A rhubarb isolate of CLRV was distinguished serologically from a cherry isolate of the virus. Turnip mosaic virus, CLRV and SLRV, were transmitted with difficulty, but AMV isolates were readily transmitted by mechanical inoculation. Turnip mosaic virus was also transmitted to rhubarb by Myzus persicae and Aphis fabae. CLRV was transmitted in 6–8% of the seed of infected ‘Prince Albert’ and ‘Victoria’ rhubarb and in 72% of the seed of infected Chenopodium amaranticolor. Mild isolates of AMV were also transmitted in 10–24% of the seed of infected ‘Prince Albert’ and ‘Victoria’ plants.     

Seed-transmission in the ecology of nematode-borne viruses

Virus-free populations of vector nematodes can acquire tomato black ring (TBRV), raspberry ringspot (RRV) and arabis mosaic (AMV) viruses from weed seedlings grown from virus-carrying seed. When soils from fields where nematode-borne viruses occurred naturally were air-dried to kill vector nematodes and then moistened, TBRV and RRV occurred commonly in the weed seedlings that grew, but AMV occurred only rarely. Similar tests did not detect tobacco ringspot, grapevine fanleaf or tobacco rattle viruses in weed seeds in the single soil studied in each instance, although these three viruses are also seed-borne in some of their hosts. Many weed species, when infected experimentally, readily transmit TBRV and RRV to their seed, but the viruses were much commoner in naturally occurring seed of some of these species than of others. These discrepancies between the frequency of seed-transmission of viruses from experimentally infected plants and the extent of natural occurrence of infected seed seem largely to reflect the host preferences of the vectors. Infective Longidorus elongatus kept in fallow soil retained TBRV and RRV only up to 9 weeks. When weed seeds in the soil were then allowed to germinate, the nematodes reacquired virus from the infected seedlings. Some weed species were better than others as sources of virus. Persistence of these viruses in fields through periods of fallow or fasting of the vector therefore depends on a continuing supply of infected seedlings produced by virus-containing weed seeds. This is probably less true of viruses like AMV and grapevine fanleaf, which persist for 8 months or more in their vectors (Xiphinema spp.). A few seeds containing TBRV and RRV were found in soils free of vector nematodes, suggesting that the viruses are disseminated in weed seed. This probably explains how TBRV and RRV have reached a large proportion of L. elongatus populations in eastern Scotland.     

Seed-transmission of nematode-borne viruses

Transmission through seed of crop and weed plants seems to be characteristic of nematode-borne viruses. It occurred with tomato black ring virus (TBRV) in nineteen species (thirteen botanical families), with arabis mosaic virus (AMV) in thirteen species (eleven families), with raspberry ringspot virus (RRV) in six species (five families), and also, in more limited tests, with tomato ringspot, cherry leaf roll and tobacco rattle viruses. A remarkable feature was that infected seedlings, except those containing tobacco rattle virus, often appeared healthy. The occurrence and extent of seed-transmission depended on both the virus and the host plant. In many progenies more than 10%, and in some 100%, of seedlings were infected. The viruses were transmitted through at least two or three generations of seed of those host species tested. After 6 years' storage, TBRV- and RRV-containing seed of Capsella bursa-pastoris and Stellaria media germinated to give infected seedlings. In controlled crossing experiments with strawberry and raspberry, virus was transmitted to seed from both male and female parents but, at least in raspberry, the presence of competing virus-free pollen much decreased the ability of pollen from infected plants to set seed. There was no evidence that healthy mother plants became infected when their flowers were pollinated with infected pollen.     

SEED DISPERSAL OF METROSIDEROS POLYMORPHA (MYRTACEAE): A PIONEER TREE OF HAWAIIAN LAVA FLOWS

Wind dispersal of seeds of Metrosideros polymorpha, the dominant tree of rain forests and the main pioneer of volcanic substrates in Hawaii, was measured on a 20-yr-old lava flow downwind of a M. polymorpha forest. Seed rain was concentrated in December and January, when 74.5% of the annual total was dispersed. Annual seed rain at the forest edge was 63,893 seeds/m², of which 5,580 (8.7%) contained embryos and were potentially viable. Beyond the forest edge, the density of embryo-containing seed decreased to 363, 137, 37, 25, and 20/m² at distances of 25, 50, 100, 150, and 250 m, respectively. In contrast, the density of established M. polymorpha seedlings remained relatively constant beyond 25 m, a fact suggesting that the density of safe sites for germination and establishment was also constant over the same distance. It was concluded that colonization of the lava flow was limited by the density of safe sites, i.e., conditions for establishment, rather than by seed rain, i.e., dispersal, which was more than sufficient to saturate the available safe sites.     

Healthier seed potatoes. II. Effects of sprouting and benomyl treatment of stem cutting and commercial stocks on growth, yield and disease

Potato seed tubers of seven cultivars derived from stem cuttings in 1965 (healthier seed) were compared with samples from commercial stocks in 1969–72 at two sites, one clay with flints soil (Rothamsted) and the other sandy loam soil (Woburn). Plant emergence, usually slower from non-sprouted seed, was unaffected by seed source or seed treatment with benomyl. Sprouted healthier seed produced more stems/plant than sprouted commercial seed. Healthier seed yielded 6% (Rothamsted 5%, Woburn 8%) more than commercial when seed was sprouted and 7% (Rothamsted 6%, Woburn 9%) when not sprouted. The increased yields of cvs Record (10% sprouted, 23% not sprouted), Majestic (8% sprouted, 11% not sprouted) and King Edward (5% not sprouted) mostly comprised small ware (44–57 cm) and chats (>44 cm), indicating an increase in tuber numbers. Commercial Pentland Crown seed yielded as much as healthier but the healthier produced less large ware (57–83 cm). Benomyl treatment of commercial seed, especially when not sprouted, and sometimes of healthier seed also decreased tuber size. Infection of stem bases and tuber eyes by Polyscytalum pustulans was less from healthier than commercial seed and was decreased by benomyl. Stem canker (Rhizoctonia solani) was also decreased by benomyl but fungicide treatment of seed did not greatly decrease the high incidence of R. solani hyphae on eyes of tubers at Woburn. Helminthosporium solani, equally prevalent on the produce of commercial and healthier seed, was decreased by benomyl. There was slightly more gangrene (Phoma exigua) on the produce of commercial than healthier seed.     

Transmission of the hop strain of arabis mosaic virus by Xiphinema diversicaudatum*

Hop plants became infected with the hop strain of arabis mosaic virus (AMV(H)) when grown in hopfield and woodland soil in which infected plants had been growing. Infection occurred in soil infested with the dagger nematode Xiphinema diversicaudatum, but neither in uninfested soil nor in soil previously heated to kill nematodes. X. diversicaudatum transferred direct from hop soils transmitted AMV(H) to young herbaceous plants and to hop seedlings; some of the hop seedlings developed nettlehead disease. A larger proportion of plants was infected using X. diversicaudatum obtained from a woodland soil and then given access to the roots of hop or herbaceous plants infected with AMV(H). AMV(H) was transmitted by adults and by larvae, in which the virus persisted for at least 36 and 29 wk, respectively. Difficulties were encountered in detecting AMV(H) in infected hop plants, due partly to the delay in virus movement from roots to shoots. Infection of hop shoots was seldom detected until the year after the roots were infested and sometimes nettlehead symptoms did not appear until the third year. Isolates of arabis mosiac virus from strawberry did not infect hop. The results are discussed in relation to the etiology and control of nettlehead and related diseases of hop.     

Fungus diseases on potato seed tubers planted in England and Wales, 1963-76

During 1963-76 samples of potato tubers from commercial seed stocks of cvs King Edward (14 yr), Pentland Crown (9 yr), Majestic (7 yr), Pentland Dell (3 yr), Record and Arran Pilot (2 yr) were received from farms in England and Wales. Fifty tubers from each sample were examined macroscopically for fungus diseases and eyes were excised from a 20-tuber sub-sample, incubated and examined for pathogenic fungi; 50 tubers were stored on trays to sprout and examined for diseases and sprouting in May and in most years samples of 50 tubers were wounded by dropping onto expanded metal, stored at 5° C and examined for gangrene and dry rot after 12 wk. Amounts of disease varied between years and during 14 yr black scurf and powdery scab on King Edward tended to increase and skin spot and late blight decrease. On average 44% of King Edward tubers were affected with skin spot, 25% with black scurf and 16% with powdery scab. Gangrene affected 5% of tubers and 97% of the isolates from rots were identified as Phoma exigua var. foveata. Wounding tubers increased the incidence of gangrene three-fold. During 1963-69 late blight affected 2% of King Edward tubers but fewer in later years and in other cultivars. Majestic had most common scab (44% tubers) and Arran Pilot most dry rot (9% tubers) and this disease was increased by wounding tubers. Conidiophores of Helminthosporium solani (silver scurf) were more common on excised eyes of Pentland Crown, Record and Arran Pilot than of other cultivars, and isolations from verticillate conidiophores that developed on the side of incubated eye plugs of King Edward and Majestic stocks gave pure cultures of Verticillium tricorpus (78%), V. nigrescens (9%) and V. nubilum (3%). Proportions of tubers with different diseases were affected by their country of origin; Scottish seed had most skin spot and gangrene, Irish seed most powdery scab and English seed most common scab, late blight and H. solani. There was also evidence of differing disease incidence in seed from different geographical areas in Scotland and England. Up to half the King Edward and Pentland Crown stocks examined in 1975 and 1976 were derived from stem cuttings and average amounts of diseased tubers were similar to those in stocks not derived from stem cuttings. Annual and cultivar differences in disease incidence and effects of date of receipt of seed on farms are discussed.     

Selection for immunity in swede (Brassica napus) to infection by turnip mosaic virus

Fully heritable immunity to turnip mosaic virus (TuMV) was found in swede (Brassica napus) cvs Ruta Otofte and Bangholm. Attempts to ‘break’ this immunity were unsuccessful. The reactions of two immune lines of cv. Ruta Otofte to cauliflower mosaic virus, clubroot (Plasmodiophora brassicae) and powdery mildew (Erysiphe cruciferarum) were unchanged compared with the unselected parent line. A symptomless response to TuMV inoculation (possibly immunity) was also found in cv. Acme but not in the clubroot resistant cv. Marian. The percentage of plants which gave a symptomless response to TuMV inoculation differed greatly between cultivars and some seed stocks.     

Role of Seed in Survival and Transmission of Xanthomonas campestris pv. oryzae Causing Bacterial Blight of Rice

Survival period and possibility of seed transmission of X. campestris pv. oryzae were studied. The bacterium survived for longer (170–180 days) in kharif than rabi (120–130 days) harvested seed. The percentage of infected seeds was higher in kharif than rabi. The infected seed when sown failed to produce the symptoms on respective seedlings due to the low number of bacterial population. Present studies indicated that infected seed though may not produce symptoms on the seedlings directly but serve as a source of inoculum form season to season.     

Healthier seed potatoes. I. Effects of inoculating stem cutting stocks with Polyscytalum pustulans and Rhizoctonia solani on growth, yield and disease

Potato seed tubers of seven cultivars derived from stem cuttings in 1965 (healthier seed) were grown in 1969–72 at two sites, one clay with flints soil (Rothamsted) and the other sandy loam soil (Woburn). Inoculating sprouted tubers at planting with Polyscytalum pustulans did not affect the number of stems/plant or total yield but increased stem base and tuber infection. The yields of large tubers (57–83 cm) were increased and small tubers (>57 cm) decreased, indicating a decrease in tuber numbers. Rhizoctonia solani inoculated at planting decreased numbers of stems/plant and yield by up to 14% at Woburn but not at Rothamsted. At both sites, yields of large tubers were increased with cvs Majestic and Record and decreased with Pentland Crown. Stem canker and tuber infection were increased but infection was also prevalent on tubers from non-inoculated seed at Woburn. When both pathogens were inoculated together yields of large tubers were increased in cvs King Edward, Majestic and Record and decreased in Pentland Crown. Infection of stem bases and tubers was sometimes less than when either pathogen was inoculated singly.