Control of Penicillium expansum by plant volatile compounds

Nine plant volatiles were tested for their activity in vitro and in vivo against Penicillium expansum , the cause of blue mould of pear. In vitro spore germination and mycelial growth assay showed a consistent fungicidal activity by trans -2-hexenal, carvacrol, trans -cinnamaldehyde and citral, while hexanal (-)- carvone, p -anisaldehyde, eugenol and 2-nonanone exhibited a progressively lower inhibition. trans -2-Hexenal was the best inhibitor of conidial germination [MIC (minimum inhibitory concentration) = 24·6  µ L L⁻¹; ED₅₀ = 10·2  µ L L⁻¹], while carvacrol was the best inhibitor of mycelial growth (MIC = 24·6  µ L L⁻¹; ED₅₀ = 9  µ L L⁻¹). The four most active compounds in in vitro studies were tested in vivo as fumigants against blue mould on pear cv. Conference. Best control was achieved by trans -2-hexenal vapour treatments (12·5  µ L L⁻¹) when applied over a 24-h period, beginning 24 h after inoculation. In contrast, carvacrol (12·5–200  µ L L⁻¹), and trans -cinnamaldehyde (50–400  µ L L⁻¹) were ineffective and citral (200  µ L L⁻¹) showed only slight effect.     

Visual and PCR assessment of light leaf spot (Pyrenopeziza brassicae) on winter oilseed rape (Brassica napus) cultivars

Methods to assess light leaf spot ( Pyrenopeziza brassicae ) on winter oilseed rape cultivars were compared in laboratory, controlled-environment and field experiments. In controlled-environment experiments with seedling leaves inoculated at GS 1,4, the greatest differences in percentage area affected by P. brassicae sporulation were observed with inoculum concentrations of 4 × 10³ or 4 × 10⁴ spores mL⁻¹, rather than 4 × 10² or 4 × 10⁵ spores mL⁻¹, but older leaves had begun to senesce before assessment, particularly where they were severely affected by P. brassicae . In winter oilseed rape field experiments, a severe light leaf spot epidemic developed in 2002/03 (inoculated, September/October rainfall 127·2 mm) but not in 2003/04 (uninoculated, September/October rainfall 40·7 mm). In-plot assessments discriminated between cultivars best in February/March in 2003 and June in 2004, but sometimes failed to detect plots with many infected plants (e.g. March/April 2004). Ranking of cultivar resistance differed between seedling experiments done under controlled-environment conditions and field experiments. The sensitivity of detection of P. brassicae DNA extracted from culture was greater using the PCR primer pair PbITSF/PbITSR than using primers Pb1/Pb2. P. brassicae was detected by PCR (PbITS primers) in leaves from controlled-environment experiments immediately and up to 14 days after inoculation, and in leaves sampled from field experiments 2 months before detection by visual assessment.     

Fungicide sensitivity in Swedish isolates of Phaeosphaeria nodorum

This is the first report of variability in sensitivity of Phaeosphaeria nodorum to the fungicide azoxystrobin, and also reports on sensitivity to propiconazole, prothioconazole and cyprodinil. An in vitro sensitivity test of 42 isolates from five Swedish winter wheat fields, collected in 2003–2005, was performed on malt extract agar amended with six concentrations of each fungicide. Four isolates collected during the early 1990s, before azoxystrobin had been commercially used in agriculture, were used as references. Fragments of DNA from 231 isolates, including the reference isolates, were sequenced for the genes of cytochrome b and CYP51 in search for amino acid substitutions known to cause loss of sensitivity to strobilurins and triazoles, respectively. The majority of the P. nodorum isolates possessed the amino acid substitution G143A, associated with loss of sensitivity in fungi to strobilurins, except in one field where only half of the isolates had the substitution. The EC₅₀ values varied between 0·66 mg L⁻¹ to estimations far above 1000 mg L⁻¹, with an estimated median value of 366 mg L⁻¹. The EC₅₀ values of the reference isolates ranged from 0·02 to 80·72 mg L⁻¹. The P. nodorum population is still sensitive to propiconazole, prothioconazole and cyprodinil, even though some isolates varied in sensitivity to triazoles. Part of the CYP51 gene, a possible target for triazole sensitivity, was sequenced but no nonsynonymous substitutions were found.     

Sensitivity of fungi from cereal roots to fluquinconazole and their suppressiveness towards take-all on plants with or without fluquinconazole seed treatment in a controlled environment

The take-all fungus, Gaeumannomyces graminis var. tritici , was highly sensitive to fluquinconazole ( in-vitro EC₅₀ 0·016–0·018 mg L⁻¹), a fungicide developed for use as a seed treatment to control take-all, and to prochloraz (EC₅₀ 0·006 mg L⁻¹). Fungi of other genera that were commonly isolated from cereal roots were sensitive in varying degrees to prochloraz but were relatively insensitive (e.g. Fusarium culmorum , EC₅₀ > 20 mg L⁻¹) or slightly sensitive (e.g. Epicoccum purpurascens , EC₅₀ 0·514 mg L⁻¹) to fluquinconazole. Gaeumannomyces graminis var. graminis and G. cylindrosporus , weak parasites that can protect roots against take-all, and an unnamed Phialophora sp., all closely related to the take-all fungus, were highly or moderately sensitive to fluquinconazole. Alternaria infectoria and E. purpurascens were most consistently effective in suppressing development of take-all on pot-grown wheat plants dual-inoculated with G. graminis var. tritici and the nonpathogen. Take-all was decreased more on dual-inoculated wheat plants grown from seed treated with fluquinconazole or fluquinconazole plus prochloraz than when only an antagonistic fungus ( A. infectoria , E. purpurascens , Fusarium culmorum or Idriella bolleyi ) or a seed treatment was applied. These fungi were less effective in combination with seed treatments on barley. Gaeumannomyces graminis var. graminis and G. cylindrosporus , tested on wheat, suppressed take-all only in the absence of fungicides. It is suggested that the performance of seed treatment containing fluquinconazole against take-all may in some circumstances be enhanced by its partial specificity for the take-all fungus.     

A semiselective medium for the isolation of Acidovorax valerianellae from soil and plant debris

Bacterial black spot of corn-salad ( Valerianella locusta ) caused by Acidovorax valerianellae appeared in France in the beginning of the 1990s. The disease is now widespread in the main area of corn-salad production and is responsible for high depreciation and significant economic losses every year. A semiselective medium is described for the detection of the bacterium in various environments and to identify primary sources of inoculum. The medium TSAV (tryptic soy broth, 3 g L⁻¹, agar 15 g L⁻¹, 5-fluorouracyl 5 mg L⁻¹, novobiocin 5 mg L⁻¹, propiconazole 5 mg L⁻¹) increased chances of obtaining cultures of A. valerianellae from naturally infested soil and from root-debris. Using TSAV, the pathogen was detected in root-debris up to 39 days after the harvest of a diseased crop. As only a few days usually separates harvest from the next sowing date, soil is likely to be an important source of inoculum for the next crop.     

Suppression of clubroot disease under neutral pH caused by inhibition of spore germination of Plasmodiophora brassicae in the rhizosphere

To elucidate the mechanism of clubroot suppression under neutral soil pH, a highly reproducible germination assay system under soil culture conditions was designed based on the hypothesis that germinated spores of Plasmodiophora brassicae could be identified by the absence of a nucleus (i.e. having released a zoospore to infect a root hair of the host plant). Brassica rapa var. perviridis seedlings were inoculated with a spore suspension of P. brassicae at a rate of 2·0 × 10⁶ spores g⁻¹ soil and grown in a growth chamber for 7 days. The spores were recovered from rhizosphere and non-rhizosphere soils and stained with both Fluorescent Brightener 28 (cell-wall-specific) and SYTO 82 orange fluorescent nucleic-acid stain (nucleus-specific stain). Total numbers of spores were counted under UV-excitation, and spores with a nucleus that fluoresced orange under G-excitation were counted. The significant increase in the percentage of spores without a nucleus (germinated spores) in the rhizosphere after 7 days' cultivation and the correlation with root-hair infections validated the assay system. Applications of calcium-rich compost or calcium carbonate to neutralize the soil significantly reduced the percentage of germinated spores in the rhizosphere, as well as the number of root-hair infections. The present study provides direct evidence that the inhibition of spore germination is the primary cause of disease suppression under neutral soil pH.     

Pathology of cortical invasion by Plasmodiophora brassicae in clubroot resistant and susceptible Brassica oleracea hosts

The differences in physiological response of Brassica oleracea to infection and growth of Plasmodiophora brassicae in infected tissue were studied in clubroot resistant and susceptible hosts, grown in sand solution culture artificially inoculated with spores of P. brassicae (10⁸ spores mL⁻¹). Primary (root hair) and secondary (cortical) stages of P. brassicae occurred in both resistant and susceptible hosts. Symptoms of cortical invasion by P. brassicae in resistant and susceptible hosts included cell wall breaks, presence of vesicles or inclusion bodies within the cell walls, cell wall thickening in association with plasmodesmata and enlarged and/or disorganized host nuclei. The main difference between the resistant and susceptible host reaction was the absence of degradation of the secondary thickening and cell walls of the xylem in the resistant host. This study supports the existence of an amoeboid form of the pathogen in addition to the recognized two-phase life history of P. brassicae. Furthermore, it suggests that resistance in B. oleracea does not prevent the development of this amoeboid form. A reduced number of cell wall breakages suggests movement of the amoeboid form, may have been restricted but not prevented in the resistant host.     

Sensitivity to cymoxanil in populations of Plasmopara viticola in northern Italy

In 1993, control failures were reported on grapevine in northern Italy under severe downy mildew pressure after postinfection application of cymoxanil in mixtures with copper or mancozeb. A monitoring survey was started immediately in Piedmont (north-western Italy) in order to determine the sensitivity of populations of Plasmopara viticola to cymoxanil from those vineyards where the fungicide was not controlling the disease satisfactorily. In 1994 and 1995, monitoring surveys were extended to north-eastern Italy, where cymoxanil mixtures were not performing as well as in the past. Sampled populations were tested on detached leaf discs and on whole potted plants under controlled conditions. In 1993, 12 populations, sampled in Piedmont, showed MIC values (minimum inhibitory concentration) varying from 10 to more than 100 mg L⁻¹ cymoxanil. With a baseline reference population having a MIC value of 3 mg L⁻¹, resistance factors ranged from 3 to more than 30. In 1994, 17 populations out of 27 sampled in Trentino (north-eastern Italy) showed MIC values of 100 mg L⁻¹ or higher and in 1995, 32 populations out of 38 showed the same behaviour. In similar experiments, the MIC values of populations from nontreated plots were between 3 and 10 mg L⁻¹. In whole potted plant tests, populations with MIC values higher than 200 mg L⁻¹ in a leaf disc test were not controlled by 500 mg L⁻¹ of cymoxanil. The results of our study suggest that resistance to cymoxanil in P. viticola may contribute to a lack of disease control in Italian vineyards.     

Aggressiveness and host specificity of Brazilian isolates of Phytophthora infestans

The population of Phytophthora infestans in Brazil consists of two clonal lineages, US-1 associated with tomatoes and BR-1 associated with potatoes. To assess whether host specificity in these lineages resulted from differences in aggressiveness to potato and tomato, six aggressiveness-related epidemiological components – infection frequency (IF), incubation period (IP), latent period (LP), lesion area (LA), lesion expansion rate (LER) and sporulation at several lesion ages (SSLA) – were measured on detached leaflets of late blight-susceptible potato and tomato plants. Infection frequency of US-1 was similar on potato and tomato leaflets, but IF of BR-1 was somewhat reduced on tomato. Incubation period was longer on both hosts with US-1, although this apparent lineage affect was not significant. Overall there was no host effect on IP. On potato, BR-1 had a shorter LP (110·3 h) and a larger LA (6·5 cm²) than US-1 (LP = 162·0 h; LA = 2·8 cm²). The highest LER resulted when isolates of BR-1 (0·121 cm² h⁻¹) and US-1 (0·053 cm² h⁻¹) were inoculated on potato and tomato leaflets, respectively. The highest values of the area under the sporulation capacity curve (AUSC) were obtained for isolates of US-1 inoculated on tomato leaflets (6146) and for isolates of BR-1 on potato leaflets (3775). In general, higher values of LA, LER, SSLA and AUSC, and shorter values of LP were measured when isolates of a clonal lineage were inoculated on their original host than with the opposite combinations. There is evidence that there are quantitative differences in aggressiveness components between isolates of US-1 and BR-1 clonal lineages that probably contribute to host specificity of P. infestans populations in Brazil.     

In vitro somatic growth and reproduction of phenylamide-resistant and -sensitive isolates of Phytophthora erythroseptica from infected potato tubers in Idaho

Pink rot of potato, most commonly caused by Phytophthora erythroseptica , is a major field and post-harvest problem in southern Idaho, USA, particularly since 1998 when isolates resistant to the phenylamide fungicide metalaxyl-M (mefenoxam) were detected. Isolates of P. erythroseptica were collected from infected tubers in 2001 and 2002 from six Idaho counties and tested for resistance to metalaxyl-M on amended agar. Metalaxyl-M resistant (MR) and metalaxyl-M-sensitive (MS) isolates were identified in six counties; 160 isolates were highly resistant, seven moderately resistant and 57 sensitive to metalaxyl-M with mean EC₅₀ values of 182, 23 and 0·5 mg L⁻¹ ai metalaxyl-M, respectively. Mycelial growth rates and oospore production in agar were assessed for 20 MS and 20 MR isolates at 10, 15, 20, 25 and 30°C. Growth rates of MR isolates were between 2·5 and 3·1 times greater ( P  < 0·05) than those of MS isolates at 10, 15, 20 and 25°C, and oospore production was between 6·8 and 20·5 times greater ( P  < 0·0001) for MR than for MS isolates at the same temperatures. Colony growth in V8 broth at 18°C was greater for MR than MS isolates ( P  < 0·0032). However, zoospore production at 18°C was greater for MS than for MR isolates ( P  < 0·0109), and zoospore production m m ⁻¹ of colony circumference was also greater for MS than for MR isolates, 14 191 and 9959, respectively ( P  = 0·0109). Sexual reproduction of MR isolates in nature may be greater than MS isolates, but MS isolates may be more asexually fit based on the fitness parameters studied.     

Host–parasite relationships of Meloidogyne incognita on spinach

Host–parasite relationships in root-knot disease of spinach caused by Meloidogyne incognita race 1 were studied under glasshouse conditions. Nematode-induced mature galls were large and usually contained one or more females and egg masses with eggs. Feeding sites were characterized by the development of giant cells containing granular cytoplasm and many hypertrophied nuclei. The cytoplasm in these giant cells was aggregated alongside the thickened cell walls. Stelar tissues within galls appeared disorganized. The relationship between initial nematode population density ( P _i) in a series from 0–128 eggs and second-stage juveniles per cm³ soil and growth of spinach cv. Symphony F₁ seedlings was tested under glasshouse conditions. A Seinhorst model [ y = m  + (1 −  m ) z ^P–T ] was fitted to fresh top- and total plant-weight data for inoculated and control plants. Tolerance limits ( T ) of spinach cv. Symphony F₁ to M. incognita race 1 for fresh top and total plant weights were 0·25 and 0·5 eggs and second-stage juveniles per cm³ soil, respectively. The minimum relative values for fresh top and total plant weights were zero in both cases at P _i ≥ 32 eggs and second-stage juveniles per cm³ soil. Root galling was least at low initial population densities and greatest at 16 eggs and second-stage juveniles per cm³ soil. Maximum nematode reproduction rate was 33·1-fold at the lowest P _i.     

Resistance to Sphaerotheca pannosa in roses induced by 2,6-dichloroisonicotinic acid

The susceptible rose cv. Madelon and the partially resistant cv. Sonia both responded with reduced development of rose powdery mildew when they were treated with the synthetic inducer 2,6-dichloroisonicotinic acid (INA). The EC₅₀ for number of colonies cm⁻² was approximately 0.4 mg L⁻¹ in both cultivars when treated 4 days prior to inoculation. However, conspicuous differences were observed with respect to number of spores per cm². For sporulation, the EC₅₀ was 0.37 mg L⁻¹ in cv. Madelon and only 0.08 mg L⁻¹ in cv. Sonia. A comparison with the pathosystems cucumber/ Sphaerotheca fuliginea and red cabbage/ Peronospora parasitica is made and the importance of the observed phenomenon for the selection of parents in a breeding programme for (partial) resistance is discussed.     

Effects of benzothiadiazole and acetylsalicylic acid on β-1,3-glucanase activity and disease resistance in potato

Benzothiadiazole (BTH), as Bion WG50, and acetylsalicylic acid (ASA) treatments of potato foliage of field- and glasshouse-grown potato plants were compared for control of two foliar diseases, early blight ( Alternaria solani ) and powdery mildew ( Erysiphe cichoracearum ). The effect of these treatments on harvested tubers wound-inoculated with the dry rot fungus ( Fusarium semitectum ) was also evaluated. BTH (50 mg a.i. L⁻¹) gave almost complete control of both foliar pathogens on inoculated glasshouse-grown plants and reduced the severity of leaf spotting diseases (mainly early blight) in the field. BTH (100 mg a.i. L⁻¹) and ASA (400 mg a.i. L⁻¹) reduced the severity of dry rot in field-grown tubers in some post-harvest wound-inoculated treatments but not others and a similar reduction occurred with tubers inoculated post-harvest from BTH-treated plants grown under glasshouse conditions. BTH treatment increased β-1,3-glucanase activity in leaves > stem > tubers > stolons but not in roots. Increased enzyme activity was recorded for up to 45 days post-treatment.     

Sensitivity of Sclerotinia homoeocarpa to demethylation-inhibiting fungicides in Ontario, Canada, after a decade of use

In late 2003, nine populations of Sclerotinia homoeocarpa in Ontario Canada (seven of which had been previously sampled in early 1994, prior to the registration of sterol demethylation-inhibiting (DMI) fungicides for turf disease control in Canada) were sampled and tested for sensitivity to propiconazole. Four of the nine populations had not been treated with DMI fungicides during the intervening years, and isolates from these locations were sensitive to propiconazole (geometric mean EC₅₀ values of 0·005–0·012 µ g mL⁻¹, compared with 0·005–0·008 µ g mL⁻¹ for the original 1994 populations). Among the five populations from 2003 that had been exposed to DMI fungicides, mean EC₅₀ values were significantly greater, ranging from 0·020 to 0·048 µ g mL⁻¹. A significant correlation of determination was found between estimated number of fungicide applications and log EC₅₀ ( R ² = 0·832, P  = 0·0001), and the equation predicted that 42·3 applications of propiconazole would be needed to bring a sensitive population (EC₅₀ < 0·01  µ g mL⁻¹) to a resistant level (EC₅₀ > 0·10  µ g mL⁻¹). Fungicide sensitivity vs. duration of fungicide efficacy was also tested, and it was found that isolates with decreased sensitivity were able to more quickly overcome the inhibitory effects of fungicide application, reducing the duration of control from 3 weeks to 2 weeks.     

Reduced sensitivity of Cercospora beticola isolates to sterol-demethylation-inhibiting fungicides

In a survey conducted during October 1995, single-lesion isolates of the sugar beet leaf-spot fungus, Cercospora beticola , were tested for sensitivity to the sterol demethylation inhibiting fungicides (DMIs) flutriafol and bitertanol. The isolates were collected from fields in three different areas of northern Greece. Fields at Serres and Imathia had been sprayed with DMIs for about 15 years to control sugar beet leaf-spot. At the third site, Amyndeon, DMI fungicides had not been used. From each area 150 isolates were tested. ED₅₀ values were calculated for individual isolates by regressing the relative inhibition of colony growth against the natural logarithm of the fungicide concentration. The mean ED₅₀ values for flutriafol for the Serres, Imathia and Amyndeon populations were 1·07, 0·73 and 0·5 µg mL⁻¹, respectively (significantly different at P  = 0·05). For bitertanol the mean ED₅₀ values for the Serres and Imathia populations were 0·72 and 0·81 µg mL⁻¹, respectively, which were not significantly different at P  = 0·05. The mean ED₅₀ value of the Amyndeon population was 0·48 µg mL⁻¹, which was significantly lower than those of the other two populations ( P  < 0·05). A cross-resistance relationship was found to exist between the two triazole fungicides tested when log transformed ED₅₀ values of 60 isolates were subjected to a linear regression analysis ( r  = 0·81).     

Pathogenicity of the root-knot nematode Meloidogyne javanica on potato

Host–parasite relationships and pathogenicity of Meloidogyne javanica on potatoes (newly recorded from Malta) were studied under glasshouse and natural conditions. Potato cvs Cara and Spunta showed a typical susceptible reaction to M. javanica under natural and artificial infections, respectively. In potato tubers, M. javanica induced feeding sites that consisted of three to four hypertrophied giant cells per adult female. Infection of feeder roots by the nematode resulted in mature large galls which usually contained at least one mature female and egg mass. In both tubers and roots, feeding sites were characterized by giant cells containing granular cytoplasm and many hypertrophied nuclei. Cytoplasm in giant cells was aggregated alongside the thickened cell walls. Stelar tissues within galls appeared disorganized. The relationship between initial nematode population density ( P ) [0–64 eggs + second-stage juveniles (J2s) per cm³ soil] and growth of cv. Spunta potato seedlings was tested under glasshouse conditions. A Seinhorst model [ y = m  + (1 −  m ) z ^{( P − T )}] was fitted to fresh shoot weight and shoot height data of nematode-inoculated and control plants. Tolerance limits ( T ) for fresh shoot weight and shoot height of cv. Spunta plants infected with M. javanica were 0·50 and 0·64 eggs + J2s per cm³ soil, respectively. The m parameter in that model (i.e. the minimum possible y -values) for fresh shoot weight and shoot height were 0·60 and 0·20, respectively, at P  = 64 eggs + J2s per cm³ soil. Root galling was proportional to the initial nematode population density. Maximum nematode reproduction rate was 51·2 at a moderate initial population density ( P  = 4 eggs + J2s per cm³ soil).     

Analysis of resistance to Xylella fastidiosa within a hybrid population of Pera sweet orange × Murcott tangor

Resistance to Xylella fastidiosa was evaluated within a population of 20 interspecific hybrids of Pera sweet orange and Murcott tangor under greenhouse conditions. Efficiency of inoculation, multiplication of bacteria within the plants, xylem vessel morphology, and symptom expression were analysed. The rate of infection ranged from 40 to 100% (average 70%) for all genotypes analysed. Xylella fastidiosa populations ranged from log 0·59 to log 2·13 cells mg⁻¹ tissue for the resistant hybrids. These values were significantly different ( P  = 0·05) from those obtained for the tolerant (no symptoms but bacteria recovered) or susceptible (symptoms and bacteria recovered) hybrids (log 3·02 to log 4·06 cells mg⁻¹). Xylella fastidiosa was recovered from all hybrids (log 2·31 to 5·03 CFU mg⁻¹ tissue) except the resistant ones. The first foliar symptoms appeared at least 90 days post-inoculation, the time varying according to genotype. No correlation between xylem vessel morphology and disease expression was observed, indicating that the resistance was the result of a genetic response of the host. According to this hypothesis, a high broad-based heritability index for resistance was obtained (0·96) at 210 days from X. fastidiosa inoculations, using bacterial quantification by real-time PCR, which indicated that the influence of the number of bacteria was the result of genetic rather than environmental variations.     

Sensitivity of Botrytis cinerea from vegetable greenhouses to boscalid

Between 2004 and 2006, 228 isolates of Botrytis cinerea from two regions in China were characterized for baseline sensitivity to boscalid, a new active ingredient that interferes with succinate ubiquinone reductase in the electron transport chain. The isolates showed similar sensitivity in different years and regions. Baseline sensitivities were distributed as unimodal curves with mean EC₅₀ values of 1·07 (± 0·11) and 0·42 (± 0·05) mg L⁻¹ for inhibition of mycelial growth and conidial germination, respectively. Laboratory studies were conducted to evaluate the risk of development of resistance to boscalid. Boscalid-resistant mutants were obtained by UV-treatment at lower frequencies and with smaller resistance factors than pyrimethanil-resistant mutants. All boscalid-resistant mutants were also significantly more sensitive to Qo inhibitors than their wild-type parents and showed reduced sporulation in vitro and pathogenicity on aubergine leaves. The results suggested that the risk of resistance developing for boscalid was lower than for pyrimethanil. However, as B. cinerea is a high-risk pathogen, appropriate precautions against resistance development should be taken. Synergism between the activity of boscalid and that of kresoxim-methyl was observed.     

Factors influencing infection of mechanical wounds by Fusarium circinatum on Monterey pines (Pinus radiata)

Pitch canker, caused by the fungus Fusarium circinatum , is a disease affecting many pine tree species. In California, Pinus radiata (Monterey pine) is the principal pine host affected by pitch canker. This investigation into factors affecting infection frequency by F. circinatum of P. radiata examined the influence of: (i) wound size; (ii) relative humidity; (iii) time of inoculation; (iv) inoculum density; and (v) wound age. Wounded branches sustained significantly more infections when large-diameter (1·6 mm) rather than small-diameter (0·5 mm) wounds were made. Infection frequencies tended to be higher at 100% RH than at ambient humidity, although these differences were not statistically significant. Infection frequencies were significantly higher on branches inoculated after 17·00 h than on branches inoculated before noon. Infection frequencies were significantly higher for wounded branches spray-inoculated with 5 × 10⁷ rather than 1 × 10⁷ spores mL⁻¹. Infection frequencies of pruning wounds declined as wounds aged.     

Factors affecting the control of Pythium ultimum damping-off of sugar beet by Pythium oligandrum

Application of Pythium oligandrum to a soil-based compost as a mycelial suspension (5 × 10² CFU g⁻¹ of dry compost) and oospore alginate pellets (10⁵ oospores/g of dry compost) controlled pre- and postemergence damping-off of sugar beet caused by Pythium ultimum to a level similar to metalaxyl seed treatment. Oospore seed treatments and aqueous suspensions of oospores applied to compost failed to control disease. Problems in the use of P. oligandrum oospore inocula for the control of damping-off were highlighted. It was shown that treatment of oospores with cellulase (20 g L⁻¹) increased germination approximately three-fold in comparison to untreated spores. Untreated and cellulase pretreated oospores were subsequently evaluated as seed treatments for their ability to control damping-off of sugar beet. The highest rate of pretreated oospores (10⁴ oospores/seed) gave levels of emergence and establishment in infested compost that were not significantly different from the uninfested controls, whereas seed treatment with untreated oospores gave no significant reduction in disease. In a trial carried out in a controlled environment to assess the effect of pH (4.5–8.0), P. oligandrum (10⁴ cellulase pretreated oospores/seed) was shown to control pre- and postemergence damping-off of sugar beet at pH 7.0 and 7.5 only.