Effect of temperature and leaf wetness duration on infection of sweet oranges by Asiatic citrus canker

Asiatic citrus canker, caused by Xanthomonas smithii ssp. citri , formerly X. axonopodis pv. citri , is one of the most serious phytosanitary problems in Brazilian citrus crops. Experiments were conducted under controlled conditions to assess the influence of temperature and leaf wetness duration on infection and subsequent symptom development of citrus canker in sweet orange cvs Hamlin, Natal, Pera and Valencia. The quantified variables were incubation period, disease incidence, disease severity, mean lesion density and mean lesion size at temperatures of 12, 15, 20, 25, 30, 35, 40 and 42°C, and leaf wetness durations of 0, 4, 8, 12, 16, 20 and 24 h. Symptoms did not develop at 42°C. A generalized beta function showed a good fit to the temperature data, severity being highest in the range 30–35°C. The relationship between citrus canker severity and leaf wetness duration was explained by a monomolecular model, with the greatest severity occurring at 24 h of leaf wetness, with 4 h of wetness being the minimum duration sufficient to cause 100% incidence at optimal temperatures of 25–35°C. Mean lesion density behaved similarly to disease severity in relation to temperature variation and leaf wetness duration. A combined monomolecular-beta generalized model fitted disease severity, mean lesion density or lesion size as a function of both temperature and duration of leaf wetness. The estimated minimum and maximum temperatures for the occurrence of disease were 12°C and 40°C, respectively.     

Protection of olive planting stocks against parasitism of root-knot nematodes by arbuscular mycorrhizal fungi

The effects were investigated, under controlled conditions, of single and joint inoculation of olive planting stocks cvs Arbequina and Picual with the arbuscular mycorrhizal fungi (AMF) Glomus intraradices , Glomus mosseae or Glomus viscosum , and the root-knot nematodes Meloidogyne incognita and Meloidogyne javanica , on plant performance and nematode infection. Establishment of the fungal symbiosis significantly increased growth of olive plants by 88·9% within a range of 11·9–214·0%, irrespective of olive cultivar, plant age and infection by M. incognita or M. javanica . In plants free from AMF, infection by Meloidogyne spp. significantly reduced the plant main stem diameter by 22·8–38·6%, irrespective of cultivar and plant age. Establishment of AMF in olive plants significantly reduced severity of root galling by 6·3–36·8% as well as reproduction of both Meloidogyne spp. by 11·8–35·7%, indicating a protective effect against parasitism by root-knot nematodes. Infection by the nematodes influenced root colonization by AMF, but the net effect depended on the AMF isolate–olive cultivar combination. It is concluded that prior inoculation of olive plants with AMF may contribute to improving the health status and vigour of cvs Arbequina and Picual planting stocks during nursery propagation.     

Effects of different cultivated or weed grasses, grown as pure stands or in combination with wheat, on take-all and its suppression in subsequent wheat crops

Grass species were grown in plots, as pure stands or mixed with wheat, after a sequence of wheat crops in which take-all ( Gaeumannomyces graminis var. tritici ) had developed. Annual brome grasses maintained take-all inoculum in the soil as well as wheat (grown as a continuous sequence), and much better than cultivated species with a perennial habit. Take-all developed more in wheat grown after Anisantha sterilis (barren brome) or Bromus secalinus (rye brome), with or without wheat, than in continuous grass-free wheat in the same year, where take-all decline was apparently occurring. It was equally or more severe, however, in wheat grown after Lolium perenne (rye-grass) or Festuca arundinacea (tall fescue), despite these species having left the least inoculum in the soil. It was most severe in plots where these two grasses had been grown as mixtures with wheat. It is postulated that the presence of these grasses inhibited the development of take-all-suppressive microbiota that had developed in the grass-free wheat crops. The effects of the grasses appeared to be temporary, as amounts of take-all in a second subsequent winter wheat test crop were similar after all treatments. These results have important implications for take-all risk in wheat and, perhaps, other cereal crops grown after grass weed-infested cereals or after set-aside or similar 1-year covers containing weeds or sown grasses, especially in combination with cereal volunteers. They also indicate that grasses might be used experimentally in wheat crop sequences for investigating the mechanisms of suppression of, and conduciveness to, take-all.     

Alternaria Brown Spot of Minneola in Greece; Evaluation of Citrus Species Susceptibility

During the last three years, a new disease was observed in northwestern Greece on Minneola trees, hybrid of mandarin and grapefruit. On May small brown necrotic leaf spots surrounded by yellow halo areas of various sizes appeared and covered a major portion of the leaves with extension of necrosis into the veins. On young fruits small, slightly depressed black spots were the first symptoms, which later became 2–7 mm in diameter. Brown spots were observed on the leaves and fruits in several orchards in the same area, causing leaves and fruits to drop. In some orchards over 50% of the fruits were affected. From the fruit and leaf spots the typical small-spore species Alternaria alternata was isolated. Pathogenicity tests were performed by artificially inoculating fruits of Minneola, common mandarin and Clementine. The symptoms of the disease were reproduced only on fruits of Minneola hybrids by the specific strain of the fungus Alternaria alternata pv. citri. Different citrus susceptibility tests indicated that mandarins Minneola, Nova and Page were very susceptible to tested isolates while Clementine SRA and Poros Clementine were not. All lemons and lime Seedless were not susceptible. Grapefruit New Hall was not susceptible, while the Star Ruby was. Orange Lane Late, Navel Late, Oval Poros, Olinda, Navel Athos were not susceptible and only Moro showed reaction being slightly susceptible only to one isolate.     

Appearance of race Pfs:5 of spinach downy mildew fungus, Peronospora farinosa f. sp. spinaciae, in Japan

The races for the causal agent of spinach downy mildew Peronospora farinosa f. sp. spinaciae were identified by inoculation of race-differential cultivars. One isolate was identified as Pfs:5s and the others belonged to a new race. This is the first report of race Pfs:5 and another new race in Japan.     

Major gene resistance in Brassica napus (oilseed rape) is overcome by changes in virulence of populations of Leptosphaeria maculans in France and Australia

Resistance of Brassica napus (oilseed rape, canola) conferred by three different major resistance genes has been overcome by changes in virulence of Leptosphaeria maculans populations in France and Australia. In South Australia where B. napus cultivars with major gene resistance derived from Brassica rapa ssp. sylvestris were grown extensively, resistance was rendered ineffective within 3 years of commercial release of the cultivar. Disease severity was higher on cultivars with sylvestris-derived resistance than cultivars with polygenic resistance. This Australian situation is compared to that in France, where resistance conferred by the Rlm1 gene was overcome nation-wide in 5 years under commercial cropping practices, and also where a source of resistance introgressed into B. napus from B. juncea was rendered inefficient in 3 years in experimental field plots near Rennes.     

Pea extracellular Cu/Zn-superoxide dismutase responsive to signal molecules from a fungal pathogen

We previously reported that the release of O₂ ⁻ from isolated pea cell walls was enhanced by a 70-kDa glycoprotein elicitor but was suppressed by mucin-type glycopeptide suppressors (supprescins A and B) prepared from pycnospore germination fluid of Mycosphaerella pinodes, causal agent of Mycosphaerella blight of pea. Here, we show that superoxide dismutase (SOD) in the apoplast fluid/cell wall of pea seedlings responds to the fungal elicitor and suppressor molecules. In a pharmacological study and with internal amino acid sequencing, the apoplastic SOD in a pea cultivar Midoriusui was found to be a Cu/Zn type SOD. We cloned a full-length cDNA of the Cu/Zn-SOD and designated it as PsCu/Zn-SOD1. An increase in PsCu/Zn-SOD1 mRNA and the PsCu/Zn-SOD1 protein was induced by treatment with the elicitor more intensively than by wounding. Such induction by the elicitor or wounding, however, was inhibited by the concomitant presence of supprescins. The SOD activity of recombinant PsCu/Zn-SOD1 was regulated directly by these signal molecules in a manner similar to their effect on the SOD activity in the apoplastic fluid and in the cell wall-bound proteins. Based on these findings, we discuss a role for PsCu/Zn-SOD1 in the pea defense response. The nucleotide sequence data of PsCu/Zn-SOD1 reported are available in the DDBJ/EMBL/GenBank databases under accession number AB189165.     

Control of soil rot of sweet potato by the incorporation of the fresh aerial tissue of Geranium carolinianum L. into the soil

In a previous study, we found that a 70% aqueous ethanol extract of the fresh aerial tissue of Geranium carolinianum L. showed antimicrobial activity against the pathogen causing soil rot of sweet potato. As the appropriate time for cultivation of sweet potato and the growing period of G. carolinianum do not overlap in Okinawa Prefecture, Japan, the fresh aerial tissue is available in order to control soil rot of sweet potato. Thus, we examined the control effect of fresh aerial tissue against soil rot of sweet potato. The various trials (a single repetition of 20 m²) were performed in fields that had undergone 8 years of continuous cropping of sweet potato at Okinawa Prefectural Agricultural Experiment Station, Horticultural branch. After harvest, when the disease severity was evaluated by determining the necrotic area of the storage root, the incorporation of fresh aerial tissue (5000 kg 1000 m^-2) into the soil was considered to be highly effective, with a protective value of 75.4. This result shows that G. carolinianum could be used as a biological agent for the control of soil rot of sweet potato.     

Pathogenesis-related gene expression in rice is correlated with developmentally controlled Xa21-mediated resistance against Xanthomonas oryzae pv. oryzae

Disease resistance mediated by the resistance gene Xa21 is developmentally controlled in rice. We examined the relationship between Pathogenesis Related (PR) defense gene expression and Xa21-mediated developmental disease resistance induced by Xanthomonas oryzae pv. oryzae (Xoo). OsPR1a, OsPR1b, and OsPR1c genes were cloned and their induction was analyzed, in addition to the OsPR10a gene, at the juvenile and adult stages in response to a wildtype Xoo strain that induces a resistance response (incompatible interaction) and an isogenic mutant Xoo strain that does not (compatible interaction). We found that the adult stage leaves are more competent to express these OsPR1 genes and that the Xa21 locus is required for the highest levels of induction.