Pea seed infection by Mycosphaerella pinodes and Ascochyta pisi and its control by seed soaks in thiram and captan suspensions

Apparently normal pea seeds from pods bearing lesions of Mycosphaerella pinodes were often internally infected with the fungus. When infected seeds were sown in sterile grit almost all the seedlings showed disease lesions, at or below soil level, 4–6 weeks after sowing. Seed infected with Ascochyta pisi gave only 40% infection of seedlings: these showed lesions on the stem and first two leaves within 4 weeks of sowing. Infection of seeds by both pathogens could be eradicated by soaking the seeds for 24 hr. in 0.2% suspensions of thiram or captan at 30d?C. In laboratory or greenhouse tests these treatments did not check germination, but in the field the captan treatment reduced emergence. The treated seeds became fully imbibed but could be dried and stored: the thiram treatment was used for semi-commercial treatment of quantities of seed up to 3 cwt.     

Pathogenicity studies on isolates of Leptosphaeria maculans from brassica seed production crops in south-east England

Out of 117 isolates of Leptosphaeria maculans collected from a range of brassica seed crops in south-east England, 26 were virulent when tested on cabbage cv. January King. Of these, 24 were isolated from oil-seed rape (Brassica napus) and the remainder from a swede (B. napus) and a cabbage (B. oleracea) seed crop. Virulent strains were derived from infected stems and from single ascospores of the sexual state present on diseased stubble. In glasshouse tests virulent isolates were not host-specific and caused severe cankers on cabbage, kale, swede, turnip and oil-seed rape; under field conditions, ascospores originating from diseased oil-seed rape stubble produced severe cankers and high levels of seed-borne infection in eight horticultural and vegetable brassica hosts (B. napus, B. campestris and B. oleracea). Virulent and non-virulent types could be distinguished by certain cultural characteristics. On nutrient agar virulent types grew slowly, irregularly and quickly staled whereas non-virulent types grew rapidly, regularly and did not stale. The latter also produced a yellow-brown pigment in liquid culture. The potential for cross-infection of virulent strains originating from oil-seed rape to other brassica seed crops has serious implications for disease incidence on forage and vegetable brassica seed crops.     

Studies on the etiology of black rot, Stemphylium radicinum (Meier, Drechsl. & Eddy) Neerg., and leaf blight, Alternaria dauci (Kühn) Groves & Skolko, on carrot crops; and on fungicide control of their seed-borne infection phases

Of nineteen commercial samples of carrot seed in use at Wellesbourne in 1963, eight were found to be infected with both Stemphylium radicinum and Alternaria dauci and five with S. radicinum alone. Both fungi caused damping-off of seedlings and when carrots were grown at high densities A. dauci caused severe foliar infection. Neither fungus gave rise to marked infection of ware crops at normal spacings but, when seed infected with S. radicinum was sown in the autumn, the resulting seed crop was heavily infected by the succeeding autumn. Spring-sown carrots grown in soil that had borne infected crops or contained debris infected with either fungus showed little or no infection in the autumn or after storage for 3 1/2 months; autumn-sown or spring-planted (steckling) carrots put into ground infected with S. radicinum showed severe infection in the following autumn. This suggested that ware-crop carrots were resistant to infection from the soil whereas the seed crop was not. Both fungi were eradicated from seed by a 24 hr. soak at 30d? C. in a 0.2% thiram suspension, but were not completely eliminated by seed dust treatments with a range of fungicides.     

Inhibition of virulent and hypovirulent Cryphonectria parasitica growth in dual culture by fungi commonly isolated from chestnut blight cankers

Chestnut blight cankers, caused by the fungus Cryphonectria parasitica, are prone to invasion by other microorganisms as the canker ages. This microbial community has the potential to alter canker expansion, which may influence the probability that the canker girdles the infected stem. Hypoviruses infect the pathogen mycelium directly and are known to decrease pathogen virulence (i.e. hypovirulent). These viral infections can slow pathogen growth, decreasing the rate of canker expansion and lowering the probability of girdling. Saprophytic fungi also invade the expanding canker and may antagonize C. parasitica leading to reduced pathogen growth. The combined effects of fungal antagonism and a hypovirulent pathogen could work in combination to reduce the probability of girdling the infected stem. We assessed the ability of different fungal taxa, isolated from low severity cankers, to inhibit the growth of virulent and hypovirulent forms of C. parasitica in dual culture tests on two cultural media. Percent growth inhibition of virulent C. parasitica by potentially antagonistic fungi ranged from 2 % to 34 %, while inhibition of hypovirulent C. parasitica ranged from 18 % to 54 %. Only one isolate, identified as Umbelopsis isabellina (UmbelopsisWS) inhibited the virulent form of the pathogen more than the hypovirulent form. All three Trichoderma isolates caused the greatest growth inhibition of virulent C. parasitica, but they, like all other fungal isolates tested, inhibited the hypovirulent form of the pathogen more than the virulent form. These results suggest that commonly occurring fungi in chestnut blight cankers, including Trichoderma, may inhibit the hypovirulent C. parasitica more than virulent C. parasitica. Thus, the presence of other fungi in cankers may not enhance the effect of hypovirulent C. parasitica to delay cankers from girdling a stem but instead intensify canker development.     

Induction of male and female sterility in isabgol (<Emphasis Type="Italic">Plantago ovata</Emphasis>) due to floral infection of downy mildew (<Emphasis Type="Italic">Pernospora plantaginis</Emphasis>)

Downy mildew (Peronospora plantaginis) caused two different types of infection in the floral parts of isabgol (Plantago ovata). Systemic infection resulted in long spikes bearing weak and sterile florets, which later turned black due to saprophytic growth. Localised infection produced various symptoms ranging between normal flower opening and failure to bloom. Different parts of infected flowers such as sepal, petal, filament and anther were reduced in size compared to healthy flowers. However, gynoecium was elongated in localised infection. P. plantaginis induced gradual sterility of isabgol flowers. Androecium was affected more than the gynoecium was. Pollen number, pollen viability and germination reduced drastically due to localised infection. On the contrary, there were no significant differences between healthy and locally infected flowers in terms of stigma receptivity. In systemically infected spikes, bud development was arrested leading to sterility. When localised disease severity was high, secondary systemic infection caused similar symptoms. Microscopic observations showed presence of the pathogen in different parts of the flowers. Downy mildew adversely affected seed yield and quality; producing seeds, which were smaller and lighter than the healthy ones and later, became black. Seed yield was reduced by as much as 73.45 percent. Husk content per unit seed mass increased relatively as the total surface area of infected seeds increased.     

Host range and some properties of groundnut rosette virus

Two isolates of groundnut rosette virus from East Africa (GRVE₁ and GRVE₂) and from West Africa (GRVW₁ and GRVW₂) were transmitted by Aphis craccivora obtained from West Africa. A third isolate from West Africa (GRVW₃) was not transmitted by A. craccivora from three widely separated sources. GRVW₁, GRVW₂ and GRVW₃ caused leaf-symptoms in groundnut of a mosaic pattern in light and dark green. GRVE₁ and GRVE₂ caused chlorosis or chlorosis and leaf distortion as well as mosaic symptoms. Groundnut plants with GRVW₁ could not be infected by means of aphids with GRVE₁, and GRVE₁ gave similar protection against GRVW₁, which suggests that they are strains of the same virus. All isolates were transmissible manually from groundnut to groundnut (Arachis hypogea), Trifolium incarnatum and T. repens, and caused systemic infection. Inoculated Nicotiana clevelandii and N. rustica developed symptoms but virus could not be recovered from them. Chenopodium amaranticolor, C. hybridum and C. quinoa showed local lesions on inoculated leaves. Virus could be acquired by aphids from groundnut or Trifolium repens infected by means of aphids, but not from those infected by manual inoculation. Virus could not be recovered from T. incarnatum manually or by aphids, but was transmitted by cleft-grafting from clover to groundnut. Saps extracted in borax buffer plus zinc sulphate at pH 9 from plants infected with GRVW₁ and GRVE₁ remained infective at 18° C. for 1 week, and at — 20° C. for up to 4 weeks. Virus could be recovered from frozen leaves. Buffered saps lost infectivity when heated above 50° C. for 10 min.; most were still infective when diluted 1/10 and some at 1/100. Electron micrographs of partially purified preparations contained spherical particles 25–28 mμ in diameter. There were usually only about five per microscope field and they resembled those of some other viruses.     

Epidemiology of Pseudomonas syringae Pathovars Associated with Decline of Plum Trees in the Southwest of Germany

Plum decline was associated with Pseudomonas syringae pathovars syringae and morsprunorum in Baden‐Württemberg. The trunks of affected plum trees (Prunus domestica) were girdled by bacterial cankers resulting in sudden death of infected trees. Copper compounds that were applied extensively during leaf fall and bud burst, were not effective. A minority of P. syringae strains isolated from cankers on plum trees were moderately resistant, while most strains were sensitive to cupric ions. Invasions through blossoms, leaves and wounds during the vegetation period were limited to the infection sites and plum trees coped effectively with both P. syringae pathovars eliminating them eventually. Infections after dormancy including very rare leaf scar infections did not induce cankers on the trunk. However, infections of dormant trees through frost injuries, (pruning) wounds or non‐injurious ingress by freezing and thawing were serious, because they led to cankers girdling the trunk. Control strategies to manage plum decline have to be adapted to the disease cycle. They should concentrate on the dormant period beginning with early frosts in autumn and ending with bud burst.     

Stem canker (Rhizoctonia solani) of maincrop potatoes.

In two years, potato plants were sampled at 1- or 2- weekly intervals from plots planted with seed tubers bearing sclerotia of Rhizoctonia solani (black scurf) and with seed without sclerotia either infested or not with cultures of R. solani at planting. Sprouted King Edward seed was used in 1981 and sprouted and non-sprouted King Edward and Pentland Crown seed in 1982. In both years 60–80% of shoots from seed with sclerotia and 90% of shoots from seed inoculated at planting were affected with stem canker. Most disease developed before shoots emerged although it gradually increased later when new shoots arising both from seed tubers or as branches on shoots with damaged apices (pruned shoots) became infected before they emerged. Sprouting seed tubers bearing sclerotia decreased the disease on both cultivars but with soil-applied inoculum the disease was more severe on plants from sprouted than non-sprouted seed. Some stolons were infected by R. solani soon after they developed and incidence of infection later increased. Thirty to 50% of stolons were infected on plants from infected seed tubers and 60% on plants with soil-applied inoculum. With both cultivars and sources of inoculum about 70% of the infected stolons had their apices killed (pruned).     

Detection and Location of Seed-Borne Inoculum of Didymella bryoniae and its Transmission in Seedlings of Cucumber and Pumpkin

Of the 91 tested cucurbit seed samples from thirteen countries nine from four countries were found to be infected with Didymella bryoniae. The pathogen is located on and in the seed coat including the perisperm and in the tissue of the cotyledons. Primary seedling infection occurred on the radicle, hypoctyl and cotyledons. Infection of the radicle generally caused pre-emergence tot while infection on the hypocotyl and cotyledons developed furthere inoculum for infection of the first true leaves and the stem. Experimentally, all the isolates of D. bryoniae could infect cucumber (Cucumis sativus), oriental melon (Cucumis melo var. makuwa), pumpkin (Cucurbita pepo) and watermelon (Citrullus vulgaris) at different growth stages; the susceptibility of cucumber and pumpkin was markedly influenced by previailing humid conditions.The blotter method was found more suitable for detection of seed-borne infection than the agar plate method.     

Screening trials of materials for suppressing spore production by Gloeosporium perennans on apple trees

In a screening trial of thirteen potential spore suppressants carried out in 1965, sporing cankers on Cox's Orange Pippin shoots, induced by inoculation with Gloeosporium perennans in November 1964, were treated at bud-burst with the test materials. The most effective spore suppressant was dichlo-fluanid; this, with the next five most effective materials and six additional materials, was used in a second trial in 1966, when the greatest reduction in spore production was obtained with tetrachloro-o-cresol. A general trend towards correspondence between spore output from cankers and infection on neighbouring fruits was shown, but this could be modified by a number of factors, including changes in spore viability caused by fungicide treatment.     

Studies on the seed-borne phases of dark leaf spot Alternaria brassicicola and grey leaf spot Alternaria brassicaeof brassicas

Tests in Britain on samples of basic and commercial Brassica oleracea seed between 1976 and 1978 showed that many lots were infected with Alternaria brassicicola. A. brassicae was uncommon in basic seed in these years and in commercial seed harvested in 1976 and 1977 but was frequent in seed harvested in 1978. Most affected seeds were contaminated by surface-borne spores and mycelium of A. brassicicola but many were internally infected by the fungus situated within the seed-coat and in some seeds in the embryo tissues. Superficial contamination by the fungus declined rapidly after 2 yr in cabbage seeds stored at 10 °C, 50% r.h. but internal infection persisted for up to 12 yr. In some samples, internal infection was commonly associated with small shrivelled seeds. Surface contaminated and internally infected seeds transmitted the disease but seedling infection was more closely correlated with the latter.     

Viruses associated with chlorotic rosette and green rosette diseases of groundnut in Nigeria*

Groundnut (Arachis hypogaea) plants from Nigeria with chlorotic rosette disease contained a manually transmissible virus, considered to be a strain of groundnut rosette virus (GRV(C)). GRV(C) infected nine out of 32 species in three out of nine families. It caused local lesions without systemic infection in Chenopodium amaranticolor, C. murale and C. quinoa, and systemic symptoms in Glycine max, Nicotiana benthamiana, N. clevelandii and Phaseolus vulgaris as well as in groundnut. Some ‘rosette-resistant’ groundnut lines were also infected. GRV(C) was transmitted by Aphis craccivora, but only from groundnut plants that were also infected with an aphid-transmissible second virus, which was not manually transmissible and was considered to be groundnut rosette assistor virus (GRAV). Plants infected with GRAV contained isometric particles c. 25 nm in diameter which were detectable by immunosorbent electron microscopy on grids coated with antisera to several luteoviruses, especially with antisera to bean leaf roll, potato leafroll and beet western yellows viruses. No virus-like particles were observed in extracts from plants infected with GRV(C) alone. A single groundnut plant obtained from Nigeria with symptoms of green rosette contained luteovirus particles, presumed to be of GRAV, and yielded a manually transmissible virus that induced symptoms similar to those of GRV(C) in C. amaranticolor but gave only mild or symptomless infection of N. benthamiana and N. clevelandii. It was considered to be a strain of GRV and designated GRV(G).     

OBSERVATIONS ON APPLE CANKER

Field observations on leaf-scar canker of apple confirm its importance in the southwest of England: on a heavily infected tree more than 5% of the current year's growth may be damaged. Leaf scars may become infected by spores of Nectria galligena which are sucked into the tracheids of a freshly exposed scar where they develop in a relatively protected environment. These freshly exposed scars become much less susceptible to infection after approximately one hour.
Infections established in leaf scars may develop immediately into cankers or they may be confined by a suberized or gum barrier. Mycelium from some of these confined lesions may spread into the surrounding healthy tissue through growth cracks when the buds develop. The lesions extend throughout the winter and spring; usually the scars earliest inoculated are the first to show cankers. It seems possible that infections of this type might be controlled by eradicant sprays applied just before leaf-fall.     

Potato coiled sprout in relation to seed tuber storage treatment and to infection by Verticillium nubilum Pethybr

Inoculation of seed potato tubes with Verticillium nubilum produced infection on the underground stem bases to subsequent growth in the form of brown lesions, often accompanied by longitudinal or transverse cracking. Some inoculation tests provided further evidence of a causal relationship between infection with V. nubilum and coiled sprout, but it was not possible to define the conditions in which the fungus gave rise to coiling. Seed tubers sprouted in light before planting produced more coiling than unsprouted or desprouted tubers. Sprouting had this effect whether V. numbilum was present or not and coiling induced by sprouting and that induced by V. nubilum occurred independently. The increased coiling of stem bases from sprouted tubers was often accompanied by fasciation, whereas this symptom was not usually associated with the coiling from unsprouted or desprouted seed tubers inoculated with the fungus. Brown lesions associated with V. nubilum were readily distinguishable from those of Rhizoctonia solani but not from those caused by Oospora pustulans. Neither of these fungi was found to cause coiling. Different varieties of potatoes showed differences in the incidence of coiling in response to sprouting treatment. There was no apparent differences in varietal response to infection by V. nubilum.     

Further studies on cucumber mosaic virus infection of narrow-leafed lupin (Lupinus angustifolius): seed-borne infection, aphid transmission, spread and effects on grain yield

Four field trials were done with narrow-leafed lupins (Lupinus angustifolius) in 1988 - 1989, to examine the effect of sowing seed with 5% and 0.5% cucumber mosaic virus (CMV) infection on subsequent virus spread, grain yield and percentage of infection in harvested seed. A proportion of the CM V-infected seed failed to produce established plants and thus, plots sown with 5% and 0.5% infected seed contained 1.5-2.9% and 0.2-0.3% of seed-infected plants respectively. The rate of virus spread by aphids was faster and resulted in more extensive infection at maturity in plots sown with 5% infected seed than with 0.5% infected seed. In three trials, sowing 5% infected seed resulted in yield losses of 34 - 53% and CMV infection in the seed harvested of 6 - 13%. The spread of CMV infection resulting from sowing 0.5% infected seed did not significantly decrease yield. However, late CMV spread in these plots caused > 1% seed infection. In the fourth trial, which was badly affected by drought, CMV spread only slowly, there was no significant effect of CMV on grain yield and the percentage of infected seed harvested was 3–5 times less than that in the seed sown. When CMV-infected seed was sown at different depths, target depths of 8 and 11 cm decreased the incidence of seed-infected plants by c. 15% and c. 50% respectively compared with sowing at 5 cm. However, in glasshouse tests, treatment with the pre-emergence herbicide simazine failed to selectively cull out seed-infected plants. The field trials were colonised by green peach (Myzus persicae), blue-green (Acyrthosiphon kondoi) and cowpea (Aphis craccivora) aphids. When the abilities of these aphid species and of the turnip aphid (Lipaphis erysimi) in transmitting CMV from lupins to lupins were examined in glasshouse tests, short acquisition access times favoured transmission. With 5–10 min acquisition access times, overall transmission efficiencies were 10.8%, 9.4%, 6.1% and 3.9% for the green peach, cowpea, blue-green and turnip aphids respectively.     

Bacterial Canker of Sweet Cherry (Prunus avium) in Poland

In Polish climatic conditions cherry cankers resulting from infection by Pseudomonas morsprunorum continue development in summer, but the rate may be slower than in the period from April to June. However, from the well established, active canker developing under the dwarf shoot of the susceptible variety‘Hedelfińska’in July no P. morsprunorum were isolated. On the other hand, numerous strains of the genus Erwinia were found there which caused a hyper sensitivity reaction (HR) on tobacco leaves. From the cracks on the current-year cherry shoots, due to fresh infection and from symptomless leaves P. morsprunorum strains were isolated, always accompanied by those of the Erwinia genus in the approximate proportion 1: 1 or 1:2. The strains of Erwinia genus seemed similar to the DC and YC strains isolated by BILLING and BAKER from pear and apple trees infected by fireblight and also to the strains in group III of the bacteria isolated by CROSSE from cherry leaves. In two tests (immediately after isolation and after 4 months of storage) the strains of the Erwinia genus and 3 nonidentified isolates induced HR. At a third test, after 10 months of preservation these strains were HR-negative in contrast to the P. morsprunorum isolates. The fact that strains of the Erwinia genus inducing HR were isolated in large numbers from active cherry canker where pathogenic bacteria were not detected, may indicate that they play some role in the development of cherry tree canker.     

Studies on the seed transmission of cucumber mosaic virus in chickweed (Stellaria media) in relation to the ecology of the virus

Cucumber mosaic virus (CMV) was transmitted in the seed of infected Stellaria media plants. The rate of seed transmission varied both in manually infected plants (3–21%) and in plants grown from infected seed (21–40%). In naturally infected plants the rates of transmission found were 4–29%. Seeds recovered from field soil carried 4–5% infection and in infected seed placed in the soil the virus persisted for at least 5 months. Seed transmission of CMV also occurred in infected Lamium purpureum (4%), Cerastium holosteoides (2%) and Spergula arvensis (2%) but it could not be demonstrated in six other more common weed species in five botanical families. Seed transmission in Stellaria media occurred with a British (W) and an American (Y) strain of CMV. The virus was shown to occur in S. media pollen. The importance of CMV-infected S. media seed in the soil in relation to the epidemiology of the virus is discussed.     

The field behaviour of seed-borne Ascochyta fabae and disease control in field beans

In field sowings at Cambridge 2–15% of field bean seeds carrying Ascochyta fabae produced seedlings with leaf lesions. The fungus spread for distances up to 10 m in an average season and usually infected the new crop of seed. The amount of such infection arising from a single lot varied widely when samples were grown at different centres, presumably because of differences in local weather conditions. Seed lots with approximately 1% infected seeds seem suitable for ware crop production but little or no A. fabae can be tolerated in seed intended for multiplication. Infection in British-grown commercial seed has been greatly reduced by the selection of clean seed. Health standards adopted in the Field Bean Seed Scheme may have eliminated A. fabae from one cultivar.     

Mechanism of Cold-induced Peach Stem Infection by Pseudomonas s. pv. persicae: Water Congestion in Tissues

We have previously shown that water congestion in tissues, caused by thawing after a period of slightly negative temperatures, results in the absorption and diffusion of bacteria in the bark tissues of peach stems (Prunus persica). In the present study we demonstrate that in peach branches acclimated to cold and inoculated with the bacterium Pseudomonas syringae pv. persicae, this freezing-thawing process is sufficient to induce, with no further treatment than incubation at + 10 °C, the development of cankers very similar to those observed in orchards in the winter. The first step of the process induces a particularly effective form of contamination and thus corresponds to an epidemiological phenomenon. The diffused bacterium, then, initiates pathogenic activity in locations favorable to its development. Moreover, an additional freezing-thawing sequence can result in the extension of ongoing infection by causing diffusion of bacteria from infected tissue to healthy areas.     

Sudden death of cocoa in Papua New Guinea associated with Phytophthora palmivora cankers invaded by bark beetles

An extensive survey in 1976-82 indicated that sudden death of cocoa was widespread in Papua New Guinea with losses up to 1% per month in trees over 10-yr-old, especially in neglected plantations. Over 95% trees affected had large Phytophthora palmivora canker lesions and over 90% of the cankers were invaded by bark beetles. In an intensive single year survey of one plot 18-8% of 69 healthy trees died. Ambrosiella spp., P. palmivora and Fusarium solani were isolated from wood surrounding beetle tunnels but only the last two caused extensive lesions when inoculated into the xylem of healthy trees. The evidence obtained indicated that the sudden death syndrome is initiated by P. palmivora cankers on trunks and main branches but bark beetles are a major contributory factor and fungi associated with their tunnels may contribute to the problem. There is a risk of further outbreaks of the disease as cocoa cultivars susceptible to P. palmivora continue to be planted.