Virulence and aggressiveness of single-zoospore isolates of Phytophthora fragariae

A system for scoring the virulence of isolates of Phytophthora fragariae based on a scale of root rot from 0 ( no symptoms ) to 5 (76-100% roots roiled) on a series of strawberry cultivars is described. Thirty-two single-zoospore isolates from one field site were compared by subjecting their root rot scores to cluster analysis and this grouped them into two major clusters equivalent to physiologic races B66–3 and B66-11, Different sub-clusters of isolates of race B66-11 produced different degrees of rotting on the same hosts. Apart from differences in virulence between the sub-clusters there was some evidence for differences in aggressiveness between isolates within sub-clusters.
Increasing inoculum concentration by over 300-fold increased rotting by c . 25% but did not alter the rankings of different isolate/host combinations. Repeated passage of isolates through cultivars of differing susceptibilities did not affect their pathogenicity.     

Resistance to plum pox potyvirus in apricots1

The susceptibility of 115 apricot cultivars to plum pox potyvirus (PPV) has been examined, since 1981, in the experimental plots of the Pomology Institute at Naoussa and Skydra, Makedonia (GR). Inoculation was assured by aphids, transmitting strain PPV-M (Marcus) from naturally infected trees in adjacent peach orchards. For each cultivar, four to six trees were examined for at least 4 years. Observations on symptoms were made on leaves early in May and on fruits at maturity. Most cultivars expressed severe disease symptoms. Those without symptoms were inoculated by grafting onto heavily infected old apricot trees. The grafted shoots were tested for PPV in the following year by ELISA and on the woody indicator GF305. The cultivars which were rated as resistant after artificial inoculation and ELISA came from North America: Early Orange, Stella, NJA2, Sunglo, Veecot, Harlayne, Goldrich and Henderson. Most of these have been crossed with quality cultivars for the creation of resistant hybrids. The PPV resistance of large numbers of these apricot hybrids is now under investigation.     

Interactions among maize streak virus disease, leafhopper vector populations and maize cultivars in forest and savanna zones of Nigeria

Monthly plantings of three maize cultivars with differing resistance/susceptibility to maize streak geminivirus (MSV) and samplings of Cicadulina leafhopper vectors were made from December 1988 to April 1990 in the humid forest and from May to November 1989 in the Guinea savanna zones of Nigeria. High MSV disease incidences (43–56%) were recorded on susceptible cultivar TZB-Gusau in late-season (July-October) plantings. Hybrid 8321-21 showed significantly lower MSV disease incidence and severity than the other two cultivars, TZB-Gusau and 8329-15. Cicadulina species population peaks (5.2-13.5 leafhoppers/m²) were observed before the rains ended in savanna locations and after the end of the rains in the forest zone. Cicadulina mbila was the predominant species, whereas C. storeyi (syn. C. triangula ), C. arachidis, C. similis and C. ghaurii were less common. The proportion of viruliferous leafhoppers increased as the season progressed, reaching maxima of 15–25% at the end of the growing season. Interactions between disease incidence, Cicadulina populations, maize cultivars and rainfall are discussed.     

Phage sensitivity in relation to pathogenicity and virulence of the cotton bacterial blight pathogen of Sudan

At least two pathotypes of Xanthomonas campestris pv. malvacearum are now known to exist in Sudan. The pre-Barakat (race 1) and post-Barakat (race 2) pathogens have been shown to exhibit different host specificity. The former is pathogenic and highly aggressive on only cultivars with no resistance genes or with the B₂ and/or B₃ resistance factors, while the latter can infect the B₆ cultivars also. Race 2 in Sudan, which was previously reported to infect all the standard differentials, produced milder angular leaf spot symptoms and occasionally restricted vein infection. Moreover, it exhibited reduced growth in planta compared with race 1.
Bacteriophage studies revealed that the two races are quite distinct in their phage sensitivity. Race 1 can be lysed by only three, or rarely four, of the six phages used for typing, while race 2 is sensitive to all of them. The present study suggests that phage 7 may be the type-determining phage for race 2. Race 2 strain mutants resistant to phage 3 or 4 were found to be sensitive to phage 7 and pathogenic to both Acala and Barakat, although showing marked attenuation of virulence. However, mutants resistant to phage 2 or 7 were insensitive to all the phages and although they retained their pathogenicity to Acala, they either lost the ability to infect Barakat or produced a hypersensitive reaction. The resistance of all mutants was found to be due to failure to adsorb the homologous phage, indicating a change in the cell wall. The association of this with the attenuation of virulence suggests that bacterial wall components may function as virulence determinants in Xanthomonas campestris pv. malvacearum.     

A garlic (Allium sativum) leaf blight caused by Stemphylium vesicarium in Brazil

The causal agent of a fohar disease of garlic ( Allium sutivum ) was identified as Stemphylium resicarium (Wallr) Simm Disease symptoms were reproduced on garlic leases 8 to 10 days after inoculation in a greenhouse (temperature range 22-26 C) Onion and tomato, two previously reported hosts of S, testcarnon. were also artificially infected with the isolate from garlic. This is believed to be the first report of S. testcarnom causing a severe leaf blight of garlic in the New World area.     

Brain thelohanellosis due to Thelohanellus oculileucisci (Myxozoa: Myxosporea) in Gobio gobio

The first case of a Thelohanellus infection in the brain of gudgeons (Gobio gobio) is described. The infection was found on three localities in southern Bohemia, Czechoslovakia. The spores measured 13.2-14.4 x 6.3-7.1 micron. The reasons are discussed for which the species was identified as T. oculileucisci Trojan, 1909.     

Factors affecting seed dormancy and germination of Johnsongrass, Sorghum halepense (L.) Pers.

Seeds of Johnsongrass [Sorghum halepense (L.) Pers.] germinated to higher percentages (20–30% higher) when incubated at 28 and 35° C than at 10 or 22° C. After-ripening was accelerated by dry storage of these seeds at 50°C. Seeds pre-chilled at 6°C for 2–4 weeks followed by incubation at 28°C germinated 40–60%. Light effects on germination were related to incubation temperatures; inhibitory at 22°C; no response at 28°C; and stimulatory at 35°C. Effects of gibberellin A₃ (GA₃) also varied depending on incubation temperature, sodium hypochlorite (NaOCl) immersion and light conditions. Immersion of dry seeds in either 700 mM NaOCl, 900 mM H₂O₂ or concentrated H₂SO₄ before incubation in water was effective in breaking dormancy. This result suggests the modes of action of H₂SO₄ in the termination of dormancy may be similar to those of NaOCl and H₂SO₄ as previously suggested by Hsiao & Quick (1984), that is by modification or scarification of the hull or seed coat membranes, and also by the supply of additional oxygen to the seed.     

Phytotoxicity and persistence of chlorsulfuron as affected by activated charcoal

Two bioassay procedures, using petri-dishes and pots, based on the root growth of pregerminated maize were used to study the residual phytotoxicity of chlorsulfuron under field conditions. Both bioassay procedures appeared to be equally reproducible and sensitive with residues of chlorsulfuron being detectable from 0·25 to 10·0 ng g^-1. The results indicated that persistence, movement and phytotoxicity increased with increasing rate of chlorsulfuron, but persistence of the herbicide was shorter in wet compared to dry field conditions. As little as 1 g a.i. ha^-1 of incorporated chlorsulfuron under warm and dry field conditions caused a stunting effect on maize plants (Hybrid F₁, Damon) and reduced yield by 53% compared to untreated control plants; while 5·0 and 10·0 g a.i. ha^-1 of incorporated chlorsulfuron killed all maize plants. However, under wetter field conditions, incorporated chlorsulfuron at 1·25, 2·5 and 5·0 g a.i. ha^-1 caused a stunting effect on maize plants (Hybrid F₁, ARIS) and decreased yield by 16, 57 and 92%, respectively, compared to untreated control. Incorporation of 50 kg ha^-1 of activated charcoal inactivated completely chlorsulfuron incorporated at 1·25 and 2·5 g a.i. ha^-1 and did not affect yield of maize compared to untreated control. Higher rates of activated charcoal such as 100 and 200 kg ha^-1 also inactivated chlorsulfuron applied at 1·25–5·0 g a.i. ha^-1 and did not affect grain yield of maize. Phytotoxicité et persistance du chlorsulfuron Deux méthodes d'essais biologiques, à savoir en boîte de Petri ou en pot, Basées sur la croissance des racines de maïs prégermé ont été utilisées pour étudier la phytotoxicité résiduelle du chlorsulfuron en conditions de plein champ. Les deux méthodes sont également reproductibles et sensibles à des niveaux de détection pour les résidus de chlorsulfuron de 0,25 à 10 ng g^-1. Les résultats montrent que la persistance et la phytotoxicité augmentent avec des doses croissantes de chlorsulfuron, mais la persistance est plus courte dans des conditions de plein champ humides que séches. Une dose aussi faible que 1 g de matiére active ha^-1 de chtorsulfuron incorporé en conditions chaudes et séches a causé un effet retard sur les plants de maïs (hybride F₁, Damon) et a réduit de 53% le rendement par rapport au témoin non traité; des doses de 5 à 10 g de matiére active ha^-1 de chlorsulfuron incorporé ont tué tous les pieds de maïs. Cependant, en conditions plus humides, le chlorsulfuron incorporéà 1,25, 2,5 et 5 g de matiére active ha^-1 a causé un effet retard sur le maïs (hybride F₁ ARIS) et a réduit le rendement par rapport au témoin non traité respectivement de 16, 57 et 92%. L'incorporation de 50 kg ha^-1 de charbon actif a complément inactive le chlorsulfuron incorporéà 1,25 et 2,5 g de matiére active ha^-1 et n'a pas eu de répercussion sur le rendement par rapport au témoin non traité. Des doses plus élevées de charbon actif comme 100 et 200 kg ha^-1 ont inactivé le chlorsulfuron appliquéà 1,25–5 g matiére active ha^-1et n'ont pas affecté le poids en grain du maïs. Ueber die Beeinflussung von Phytotoxizität und Wirkungsdauer von Chlorsulfuron durch Aklivkohle Zum Studium der Residualwirkung von Chlorsulfuron unter Feldbedingungen wurden zwei Bioassaymethoden, eine in Petrischalen, die andere in Töpfen, eingesetzt. Beide Methoden basierten auf dem Wurzelwachstum von vorgekeimtem Mais. Es zeigte sich, dass beide Versuchsverfahren in gleichem Masse reproduzierbar und empfindlich und in der Lage sind Rückstände von 0,25–10,0 ng g^-1 nachzuweisen. Mit steigender Chlorsulfurondosis wurde eine zunehmende Phytotoxizität, Persistenz und Mobilität des Herbizids festgestellt. Die Persistenz war unter feuchten Feldbedingungen kürzer als bei Trockenheit. Bis zu einer unteren Grenze von 1,0 g a.i. ha^-1 verursachte inkorporiertes Chlorsulfuron, unter trockenen und warmen Feldbedingungen an Mais (Hybride F₁, Damon) Wachstumshemmungen und Erntereduktionen von 53%, verglichen mit unbehandelten Kontrollpflanzen. Unter feuchteren Bedingungen, jedoch, hatten 1,25, 2,5 and 5,0 g a.i. ha^-1 eingearbeitetes Chlorsulfuron an Mais (Hybride F₁ ARIS) Wachstumshemmungen und Ernteverluste von 16, 57 und 92% zur Folge. Die Einarbeitung von 50 kg ha^-1 Aktivkohle inaktivierte 1,25 g und 2,5 g ha^-1 inkorporierles Chlorsulfuron vollständig und hatte keinerlei negative Auswirkungen auf die Maisernte, im Vergleich zu unbehandelten Kontrollen. Höhere Mengen von Aktivkohle, wie 100 und 200 kg ha^-1, inaktivierten auch Chlorsulfuronmengen von 1,25–5 g ha^-1 und hatten keinen Einfluss auf den Kömerertrag.