Studies on the frequency distribution of Pythium-induced cavity spot of carrots

Data on Pythium-induced cavity spot from experimental plots and commercial fields were collated to study the frequency distribution of cavities on carrots. It was shown that cavities tended to occur in clusters with strong evidence that they followed a Polya-Aeppli distribution rather than the Negative Binomial distribution often associated with clustering. Possible biological interpretations of the Polya-Aeppli distribution are discussed, also their implications for estimation of inoculum potential and analysis of cavity spot field trials.     

Effect of soil cultivation and anaerobiosis on cavity spot of carrots

Cultivating between rows and narrow beds of carrots reduced the severity of cavity spot in one year and reduced the incidence and severity in single rows in a second year on land where the disorder was endemic. Growing carrots on ridges initially reduced symptoms more than inter-row cultivation. Soluble carbohydrates leached from carrot roots in vitro and the quantities increased when anaerobic conditions were imposed. Untreated and cell-free extracts of soil suspensions amended with 1% glucose and incubated anaerobically applied to carrots in the laboratory caused the outer layers of the secondary phloem to collapse resembling a cavity spot lesion. Unamended and fresh soil solutions had no effect.     

Cavity spot of carrots.

Cavity spot was induced after compacted carrot beds were watered to excess in July or August, but not after treatment in October. Roots grown in pots containingfield soil from the site of a cavity spot outbreak had fewer lesions when supplied witha minimal amount of water than when waterlogged, and when the soil was mixed withpeat instead of sand. Soft rot lesions developed on roots which had been inoculated with the field soiland sealed in pots with wax and stood in water. Anaerobic pectolytic bacteriabelonging to the genus Clostridium were isolated from the lesions and were proven tobe pathogenic to carrot roots. The lesions later resembled cavity spot after the rootswere returned to dry, aerobic conditions.     

Biochemical characterisation of Pythium spp. involved in cavity spot of carrots in France

The pathogenicity and growth rate in vivo were assessed on 27 isolates of Pythium spp. recovered from cavity spot lesions on carrots grown in various parts of northwest France. Polyacrylamide gel electrophoresis of isoesterases was used to identify the Pythium spp. involved. Slow-growing isolates were more aggressive than fast-growing ones when inoculated on carrot tap roots. Isoesterase patterns identified the slow-growing isolates as P. violae and P. sulcatum; P. ultimum and P. intermedium were identified among the less aggressive fast-growing isolate group, in which some isolates were also classed as P. sylvaticum or P. irregulare, which have similar electrophoretic profiles. The incidence of Pythium spp. associated with the disease in France is discussed in regard to cavity spot in other countries.     

Detection of Pythium violae and Pythium sulcatum in carrots with cavity spot using competition ELISA

Conventional methods indicated that Pythium violae was most commonly isolated from carrot cavity spot samples from 14 UK sites. For one site the most frequently isolated species was Pythium sulcatum. Results of similar isolation work were compared with the assay of cavity spot lesions using polyclonal antibodies, raised to P. violae or P. sulcatum, in competition ELISA. Where lesions were artificially induced the test confirmed which pathogen was causal. With cavities developed on the field-grown carrots P. violae again predominated and the ELISA confirmed this. In one sample P. sulcatum was also isolated from a small number of lesions and was not detected in ELISA. The competition ELISA did not indicate presence of either Pythium in a range of non-cavity spot lesions from which attempts at isolation were negative.     

Cavity spot of carrots - observations on a commercial crop

A single field of commercially-grown carrots was sampled in September at 16 points for cavity spot incidence. Carrot tops, root core and peel were analysed for N, P, K, Ca and Mg, and soils were analysed for organic C, total N, available P, K, Ca, Mg and Na, pH, conductivity and chloride. The incidence of the disorder varied from 0–96% within a small area of field, thus tending to rule out explanations for cavity spot based entirely on weather, genotype etc. Several types of lesion were recorded in addition to ‘typical’ cavity spot, and their incidence was found to be mutually correlated. Most of these (including splitting) were positively correlated with cavity spot, and tended to be positively related to concentrations of macroelements in the carrot peel (especially N and Ca): however, ‘scabs’ were very strikingly dissociated from cavities (both on a plot and individual root basis). This dissociation appeared to be connected with soil pH, in that scabs were most common above pH 6-5, whereas cavities were most frequent below this pH.     

黄檀丑舟蛾生物学特性及防治的研究

黄檀丑舟蛾是南岭黄檀的重要食叶害虫,该虫在福建南平一年发生6代,以蛹在疏松土壤中越冬,翌年5月初成虫开始羽化。雌虫产卵于嫩叶上,成块。每雌产卵82-306粒。幼虫5龄,各代幼虫的危害盛期;第一代5月下旬至6旬上旬。第二代6月下旬,第三代7月下旬,第四代8月中旬,第五代9月下旬,第六代11月上旬至中旬。试验表明,20%杀灭菊酯4000倍液或80%敌敌畏2000倍液对幼虫均有良好的毒杀效果。     

微红梢斑螟的生物学特性及其防治

微红梢斑螟是为害马尾松的重要害虫之一,该虫在福建明溪1年发生3-4代,以幼虫越冬,幼虫5龄,卵期3-7天左右,曲虫期24-35天,越冬代138-169天,蛹期11-18天。选用40％氧化乐果、80％敌敌畏、50％甲胺磷、4.5%氯氢萄酯,20％杀灭菊脂等农药的不同浓度,采用拉丁方设计对微红梢斑螟进行防治试验。结果表明所选用的农药中80％敌敌畏500倍液、50％甲胺礴500倍液、40％氧化乐果500、1000、1500倍液,对3-5龄幼虫的防治效果部在85％以上。20%杀灭菊酯、80%敌敌畏和柴油以2：1：50或1：2：50的比例混合,防治效果均在90％以上．     

Cavity spot of carrots: Interactions between the host and pathogen, related to the cell wall

This study investigates the structural aspects of cavity spot pathogenesis. Different Pythium spp. isolated from infected carrots, apples and melons were cultured on agar in Petri dishes and used for inoculation of uninfected carrots. Only slow-growing Pythium spp. (< 15 mm day^-1), such as P. violae and P. sulcatum caused cavity spot lesions. It is suggested that slow-growing species are able to penetrate, albeit slowly, into the plant tissue for 3 to 4 days before a hypersensitive reaction develops. Fast-growing species, however, did not cause lesions. Based on ultrastructural observations, we suggest that the following sequence of events occurs between the plant and the pathogen: The fungus infects the walls and grows for several days, during which time small amounts of wall-degrading enzymes are secreted. Phenylalanine ammonia lyase (PAL) activity and phenols increase linearly immediately upon inoculation. There was a lag phase of about 5 days before lignin began to increase linearly for about a month. Dissolution of wall components decreases the solute potential and water potential in the apoplast. Thus, water moves from the symplast into the apoplast, the turgor pressure gradually dissipates, and the cells shrink and eventually die.     

Microbiological differences between limed and unlimed soils and their relationship with cavity spot disease of carrots (Daucus carota L.) caused by Pythium coloratum in Western Australia

Application of lime (4000 kg ha^-1) to a soil used for commercial carrot production (pH 6.9) significantly (p<0.05) reduced the incidence of cavity spot disease of carrots compared to unlimed soil (pH 5.1). It significantly (p<0.01) increased soil microbial activity as measured by the hydrolysis of fluorescein diacetate and arginine ammonification. The application of lime resulted in a significant (p<0.01) increase in the total numbers of colony forming units (efu) of aerobic bacteria, fluorescent pseudomonads, Gram negative bacteria, actinomycetes and a significant (p<0.01) decrease in the cfu of filamentous fungi and yeasts compared to unlimed soil. Liming also increased the cfu of non-streptomycete actinomycetes rarely reported in similar studies. These non-streptomycete actinomycetes were estimated and isolated using polyvalent Streptomyces phages and the dry heat technique to reduce the dominance of streptomycetes on isolation plates. The non-streptomycete actinomycetes isolated included species of Actinoplanes, Micromonospora, Streptoverticillium, Nocardia, Rhodococcus, Microbispora, Actinomadura, Dactylosporangium and Streptosporangium. The numbers of actinomycetes antagonistic to Pythium coloratum, a causal agent of cavity spot disease of carrots increased in soil amended with lime. Application of lime also reduced the isolation frequency of P. coloratum from asymptomatic carrot roots grown in soil artificially infested with the pathogen, 3, 4 and 5 weeks after sowing.     

Studies on melon necrotic spot virus disease of cucumber and on the control of the fungus vector (Olpidium radicale)

Melon necrotic leaf spot virus (MNSV) caused a major outbreak of a leaf necrosis disease of hydroponically-grown cucumber plants at Humberside in 1983. The virus had c. 33 nm diam. particles which reacted serologically with MNSV antiserum of Dutch or American origin. Virus particles, which contained a single polypeptide (mol. wt 45 × 10³) and a presumed RNA species (mol. wt 1.5 × 10⁶), had a sedimentation coefficient (s_20.w) of 134 S and a buoyant density in caesium chloride of 1.35 g/cm³. The virus was mechanically transmissible, confined to species of Cucurbitaceae, transmitted by zoospores of Olpidium radicale and retained in the resting spores of the fungus. MNSV is thus both water-borne and soil-borne. O. radicale zoospores were killed in <5 min in suspensions containing 20 μg/ml of the surfactant Agral (alkyl phenol ethylene oxide). The disease did not reappear in 1984 when the cucumber crops were fed with nutrients containing 20μg/ml Agral.     

Studies on watercress chlorotic leaf spot virus and on the control of the fungus vector (Spongospora subterranea) with zinc

Watercress chlorotic leaf spot virus (WCLV) caused a yellow leaf spot disease of watercress at Pickering, Yorkshire. The virus was mechanically transmitted to and maintained in Chenopodium quinoa, C. amaranticolor and Petunia hybrida in which it caused systemic symptoms. It could not be mechanically transmitted, however, from infected C. quinoa to Chrysanthemum, Gynura aurantia, potato, tomato, watercress or nine other species of Cruciferae. WCLV could be partially-purified after extraction in weak (0.05–0.1 M) but not strong (0.5 M) phosphate or tris/HCl buffer after clarification with diethyl ether and acidification to pH 3.9–4.0. Preparations were non-infective if treated with 5% (vlv) ethanol or n-butanol or if stored at — 12°C for 1 day or heated for 10 min at 54°C. Preparations were non-infective after treatment with RNase or proteinase K but not after treatment with DNase. The virus was present in roots of diseased watercress plants which also contained the watercress crook root disease fungus Spongospora subterranea f. sp. nasturtii. Tests showed that WCLV was transmitted by S. subterranea zoospores and that it persisted in the resting spores of the fungus. The crook root disease was controlled by adding 0.3–0.5 μg Zn/ml to the inlet water supply to the crop. The water that had flowed through the crop contained 0.05–0.10 μg Zn/ml. Although this increased the zinc content of the watercress from 8–9 in untreated beds to 16–48 μg Zn/g in treated beds, this was below the tolerance recommended by the Food Standards Committee. A method is described of obtaining accurate dilutions of solutions of zinc sulphate (20% w/v ZnSO₄.H₂O) in the water supplying the crop using solutions of the red dye Ariavit Amaranth.     

Studies in the biology of metals

Conclusions Lead is absorbed by actively growing roots. This tissue is therefore permeable to the lead ion.The lead absorbed is in large part localized by deposition in the regions of growth by cell division. This suggests that the chemical conditions incident to mitosis produce substances in predominant degree which react with lead in a selective manner.     

Cavity spot of carrots: II. Cell-wall-degrading enzymes secreted by Pythium and pathogen-related proteins produced by the root cells

This study investigates the biochemical relationships between carrot roots and Pythium violae, the pathogen responsible for cavity spot (CS) disease. P. violae isolates obtained from CS lesions, cultured in Petri dishes on agar were used for inoculation of uninfected mature carrots. The fungus secreted a wide spectrum of enzymes that degraded the cellulose and pectic substances of the carrot cell walls. Cellulase and polygalacuronase (pg) showed the highest activity during the first day post-inoculation, subsequently declining. Pectin lyase (PnL), pectate lyase (PeL) and pectin methylesterase (PME) gradually increased to their highest levels of activity 14 to 30 days post-inoculation. This pattern of activity enables the penetration of the fungus through the walls of the host cells and the establishment of the hyphae. Several plant pathogen-related substances such as peroxidase, chitinase, glucanase and polyphenol oxidase were produced in the infected tissue. Peroxidase activity rose in the inoculated roots from day 1 post-inoculation. Chitinase, glucanase and polyphenol oxidase activities first appeared 3–4 days post-inoculation. At this time, two bands corresponding to chitinase at about 26 and 33 KDa and one band corresponding to glucanase at about 24 KDa could be resolved by SDS-PAGE.     

Studies on the biology of Spirorbinae (Polychaeta)

Of four species studied, tolerance of extreme temperatures in greatest in Spirorbis pagenstecheri (which extends highest on the shore) and least in S. corallinae (which always occurs immersed in water). The latter species breeds between May and August, the other three from May to October. S. borealis liberates its larvae mainly at the moon's quarters, but this fortnightly rhythm is less obvious in S. corallinae and not found in the other two species. S. corallinae differs from S. borealis in being slower to re-emerge after disturbance and in having a lesser breeding size, maximum size and life span. Growth in the laboratory is slower in S. tridentatus than in the other species. Growth of S. borealis under natural conditions is much faster in summer than in winter.