Seasonal changes in germination response of buried seeds of Orobanche crenata Forsk.

Orobanche crenata seeds, collected in Syria, Egypt and Spain, were buried in the field in Syria (all three seed lots) and Spain (only Spanish seeds) and at regular intervals exhumed and tested for germination, to investigate whether the seeds exhibit an annual dormancy/non-dor- mancy cycle. When exposed directly to the synthetic germination stimulant GR24 for 7 days at 20°C, seeds only germinated in autumn after the first rains and to a limited extent in winter. When the seeds were conditioned for 11 days at 20°C prior to exposure to GR24, germination occurred during summer and autumn, but seeds were dormant in winter and early spring. The observed seasonal pattern in germinability, in relation to rainfall and soil temperature, was largely consistent with the results of an in vitro experiment by Van Hezewijk et al. (1993), investigating the effect of conditioning temperature and conditioning period on germination capacity and the development of secondary dormancy. Moisture and temperature can therefore be considered the major factors regulating induction and alleviation of dormancy in buried O. crenata seeds. There were no basic differences in response owing to site of collection of O. crenata seeds, nor to the location where they were buried. Variations saisonnières des exigences de germination de graines enfouies d'Orobanche crenata Forsk. Des graines d'Orobanche crenata récoltées en Syrie, en Égypte et en Espagne ont été enfouies au champ en Syrie (les 3 lots) et en Espagne (seules les graines d'Espagne) puis ont été exhumées a intervalles régulier pour que leur aptitude à la germination soil évaluée. Le but était de déterminer si les graines possédaient un cycle annuel dormance/non dormance. Quand elles étaient directement exposées au stimulant de germination synthétique GR24 pendant 7 jours à 20°C, les graines ne germaient qu'à l'automne après les premières pluies et peu en hiver. Quand les graines restaient pendant 11 jours à 20°C avant leur exposition au GR24, la germination seproduisait en été et à l'automne mais les graines restaient dormantes en hiver et au début du prin-temps. Les variations saisonnières d'aptitude à la germination, liées aux précipitations et à la temperature du sol, étaient en accord avec les résultats d'une expérience in vitro de Van Hezewijk et al. (1993) concernant l'effet de la température et de la durée pendant laquelle elle est appliquée, sur l'aptitude à la germination et le développement de la dormance secondaire. L'humidité du sol et sa température peuvent ainsi être considérées comme les principaux facteurs qui induisent et lèvent la dormance de graines de O. crenata enfouies. On n'observait pas de différences importantes dues au lieu de récolte ou à l'endroit oü elles étaient enfouies. Jahreszeitliche Änderungen der Keimung von vergrabenen Samen von Orobanche crenata Forsk. Proben von in Syrien, Ägypten und Spanien gesammelten Orobanche-crenata-Samen wurden in Syrien und Proben nur spanischer Herkunft in Spanien im Freiland im Boden ausgelegt und in regelmäßigen Zeitabständen ausgegraben und auf ihre Keimfähigkeit getestet, um zu untersuchen, ob die Samen einen jährlichen Dormanz-Zyklus haben. Beim direktem Auslegen in dem synthetischen Keimungsmittel GR24 öber 7 d bei 20°C keimten die Samen nur im Herbst nach den ersten Regenfällen und in beschränktem Umfang im Winter. Wenn die Samen för 11 d bei 20°C vor dem Auslegen in GR24 vorbehandelt worden waren, keimten sie im Sommer und Herbst, aber im Winter und fröhen Fröhjahr waren sie dormant. Das jahreszeitliche Verhalten der Keimfähigkeit in Abhängigkeit von Niederschlag und Bodentemperatur stimmte weitgehend mit den Ergebnissen eines In-vitro-Versuches von Van Hezewijk et al. (1993) öber die Wirkung einer Wärmevorbehandlung und Vorbehandlungszeit auf die Keimfähigkeit und die Ausprägung sekundärer Dormanz öberein. Bodenfeuchte und -temperatur können deshalb als die wichtigsten Faktoren för die Induktion und Aufhebung der Dormanz von Orobanche-crenata-Samen im Boden angesehen werden. Herkunft und Versuchsort hatten keinen erheblichen Einfluß auf die Ergebnisse.     

Seed treatment for the control of Sphaerotheca fuliginea in muskmelon seedlings

The effects of powdery mildew (caused by Sphaerotheca fuliginea )on muskmelon seedlings and the efficacy of seed treatment with systemic fungicides were examined under controlled environmental conditions. The fresh weight of seedlings infected with powdery mildew was substantially less than that of uninoculated seedlings 5 weeks after inoculation, and the inoculated seedlings shrivelled 1 week later. The fungicides fenarimol and triadimenol applied to the seeds did not affect germination and suppressed powdery mildew effectively when applied at a rate equivalent to 8 g fungicide product per kg seed. Under conditions moderately conducive to the pathogen, fenarimol markedly reduced disease severity and its effect when applied as a seed treatment was as effective as foliar treatment. When conditions were highly conducive to the pathogen, seed treatment with fenarimol was not effective. Development of powdery mildew and efficacy of the seed treatments were related to soil type: disease developed earlier and more intensely on seedlings grown in sandy soil than those in organic medium, and seed treatment had a more pronounced effect on seedlings grown in sandy soil. The results indicate that fungicidal treatment of muskmelon seeds may offer an adequate means of reducing powdery mildew infections on seedlings.     

Influence of barley variety mixtures on severity of scald and net blotch and on yield *

Two susceptible barley cultivars and two moderately resistant advanced breeding lines were grown as pure stands and as the 11 possible equi-proportional mixtures over three field seasons, in the presence of scald and net blotch. Plots were either inoculated with infested straw, or non-inoculated and sprayed with fungicide. On average, the variety mixtures restricted leaf disease development by 12%. However, mixtures of either or both susceptible cultivar/s with the moderately resistant line 1861018 restricted disease severity by 20-32%. Generally, the mixtures did not increase yield relative to the mean of the pure stands. However, in the year of highest disease severity, yield was increased by 7% owing to mixing in the inoculated plots. In addition, one mixture consistently increased yield (mean of 4%) and one mixture had consistently lower yields (mean of 6%)) over years and inoculation treatments. Thus, careful evaluation is required in this environment and with these genotypes in order to identify mixtures with positive effects on disease control and yield.     

Sources of resistance to aflatoxin production in maize

Drought-tolerant maize genotypes (Huffman, Z08-004, Tuxpan, PH 9, NRC 5348, Chunco, Saint Croix, and Arizona) were compared in the field and laboratory to toxin-resistant GT-MAS:gk and Yellow Creole. SDS-PAGE, scanning electron microscopy of kernel cuticle, amount of kernel wax, Aspergillus flavus kernel colonization, Aspergillus ear rot, insect damage, aflatoxin production, and their relationships were examined. SDS-PAGE showed the presence of a 14 kDa trypsin inhibitor in the kernels of all genotypes except Chunco, which contains a protein of a larger molecular weight. The 14 kDa trypsin inhibitor protein content in these genotypes was higher than in GT-MAS:gk and Yellow Creole. Scanning electron microscopy revealed that Arizona, Huffman, and Chunco genotypes had abundant wax deposits on kernel surfaces and the amount of pericarp wax was equal to or above that from GT-MAS:gk and Yellow Creole. Differences in Aspergillus ear rot ratings, fungal colonization, and insect damage by corn earworm were observed in all drought-tolerant maize genotypes as well as in the controls. Kernel screening assays showed that aflatoxin B(1) levels in inoculated drought-tolerant genotypes differed significantly from those in GT-MAS:gk and Yellow Creole (LSD = 576). Aflatoxin B(1) levels in the inoculated genotypes differed significantly from those of GT-MAS:gk or Yellow Creole (LSD = 1389) when grown under drought stress conditions. Pearson correlation coefficients were significant between ear rot ratings and insect damage (r = 0.75; P = 0.01) and between Aspergillus ear rot and aflatoxin levels (r = 0.54; P = 0.05). On the basis of the parameters studied, there are indications that these genotypes were potential sources of A. flavus resistance.     

Infectivity analysis of a soybean isolate of Mungbean yellow mosaic India virus by agroinoculation

Yellow mosaic disease (YMD) of legumes endemic to South Asia are caused by begomoviruses transmitted by whiteflies. Based on molecular characterization, two distinct viruses – Mungbean yellow mosaic India virus (MYMIV) and Mungbean yellow mosaic virus (MYMV) – were found previously to be the etiological agents of YMD in legumes. Here, host range studies with a soybean isolate of MYMIV (MYMIV-[Sb]) were carried out by both whitefly transmission and agroinoculation. MYMIV-[Sb] was similar to a cowpea isolate of MYMIV (MYMIV-[Cp]) in its ability to infect cowpea, thus differing from blackgram (MYMIV) and mungbean (MYMIV-[Mg]) isolates, which do not infect cowpea. Genomic analysis of DNA A and DNA B components of these MYMIV isolates show characteristic differences in complete DNA B nucleotide sequence correlating with host range differences.     

芜菁花叶病毒不同分离物3'-cDNA片段的克隆及序列分析

 从河北、北京、山东泰安和潍坊等地区的萝卜、大白菜、甘蓝、油菜上得到8个芜菁花叶病毒(Turnip mosaic virus,TuMV)分离物,测定了它们3'-末端cDNA片段的序列。根据衣壳蛋白基因(cp)核苷酸序列可以将这8个分离物与另外2个TuMV分离物分为3组:泰安旧镇大白菜(JZBC)、甘蓝(JZGL)和萝卜(JZLB)分离物为一组;北京(BJILB)、潍坊萝卜分离物(WFLB)与引起萝卜红心病的TuMV分离物(WFLB99)为一组;泰安范镇、河北、潍坊(WFLB04)萝卜和泰安旧镇油菜(JZYC)上的TuMV分离物为一组。农壳蛋白氨基酸序列比较结果与此基本一致,所不同的是分离物WFLB99与所有分离物的差异均较大,形成单独一个分支。有必要对TuMV的变异情况和致病机理进行深入研究。     

Erwinia amylovora populations resistant to oxolinic acid in Israel: prevalence, persistence and fitness

Oxolinic acid (OA) has been the only bactericide used against fire blight in pear and quince orchards in Israel since 1998. OA-resistant Erwinia amylovora strains (Ea-OA^R) were detected in several orchards in two restricted areas in the northern Galilee region during 1999–2001. In the following years, resistant strains could not be detected in some of these locations. Documenting the fate of Ea-OA^R strains in commercial orchards at eight sites in northern Israel during 2000/03 revealed that the resistant population appeared irrespective of the number of sprays applied and the severity of the disease. The persistence of the Ea-OA^R populations varied from site to site, ranging from 4 to 20 months; these differences could be attributed to the fire blight management activities of growers. Comparative studies on the fitness of Ea-OA^R and E. amylovora strains sensitive to OA (Ea-OA^S) were conducted in vitro and in planta using two strains of each group. In four of the six comparisons, disease incidence on detached blossoms inoculated with Ea-OA^S was significantly higher than that on blossoms inoculated with Ea-OA^R. In two experiments conducted on 8-year-old pear trees grown under netting, the colonization of Ea-OA^S in blossoms, annual shoots and perennial spurs was significantly higher than that of the Ea-OA^R. In two experiments conducted on 2-year-old trees grown under netting in an experimental station, the incidence of shoots exhibiting fire blight symptoms and the rate of symptom progress within the branches were significantly higher in trees inoculated with Ea-OA^S than in those inoculated with Ea-OA^R. The results of this study suggest that OA-resistant E. amylovora strains have lower fitness than wild-type strains. These findings may have implications for fire blight management.     

Aggressiveness of Xanthomonas axonopodis pv. glycines isolates to soybean and hypersensitivity responses by other plants

Xanthomonas axonopodis pv. glycines ( Xag ) causes bacterial pustule disease which can significantly reduce the production of soybean. A collection of 26 isolates of Xag from different soybean-production areas of Thailand was shown to differ with regard to aggressiveness on soybean. They also differed in their ability to induce a hypersensitive response (HR) on four cultivars of tobacco and on other plant species including pepper, tomato, cucumber, pea and sesame. Tomato was most sensitive to HR induction by Xag . Isolate KU-P-34017 caused an HR on all the plant species tested. The minimal concentration of KU-P-34017 needed to induce HR on tobacco was approximately 5 × 10⁸ CFU mL⁻¹. A bacterium–plant interaction period of at least 2·5 h was necessary for HR, and different temperatures, relative humidity and light periods did not affect HR development. Inhibitors of eukaryotic metabolism, including cobalt chloride, lanthanum chloride and sodium orthovanadate (completely), and cycloheximide (partially) blocked the HR on tobacco, indicating the association of an active plant response. In contrast, the HR on tomato was inhibited only by cobalt chloride.