Resistance of Sorghum Line CS 621 to Rhizoctonia solani AG1-IA and Other Sorghum Pathogens

Sorghum line CS 621 was evaluated along with other lines and cultivars for resistance to Rhizoctonia sheath blight, tar spot and gray leaf spot for 3 years. CS 621 was consistently resistant to these diseases even under a heavy natural outbreak of tar spot in the breeding nursery. It was also found to be more resistant to Rhizoctonia sheath blight than the resistant lines from Japan during the 1993 to 1994 screening tests. Resistance to Rhizoctonia solani AG1-IA was expressed in the form of reduction in the severity of disease and rate of infection. Evaluation of progenies from crosses involving CS 621 and a susceptible variety, UPL Sg5, indicated that additive and dominant gene effects are important in the expression of quantitative resistance to R. solani. CS 621 could therefore serve as a source of multiple resistance genes in a breeding program for high yield and stability against sorghum diseases. Received 6 August 1999/ Accepted in revised form 18 October 1999     

Morphological and histochemical investigation of the response of Olea europaea leaves to fungal attack by Spilocaea oleagina

This study investigated the response of Olea europaea (cv. Conservolea) leaves to attack by the fungal pathogen Spilocaea oleagina. Cryostat and semithin sections of healthy and S. oleagina‐infected olive leaves were analysed histochemically for polyphenol oxidase (PPO) activity and tested for programmed cell death (PCD) induction by means of terminal deoxynucleotidyl transferase‐mediated dUTP nick end labelling (TUNEL). At all stages of infection, the fungus remained localized between the external and internal layers of cuticle without crossing the pectocellulosic layer. No PCD phenomena could be detected in plant cells at any stage of the disease. However, extensive degeneration of palisade parenchyma cells was observed in advanced infections, with massive loss of cytoplasmic contents and disappearance of cell compartments. Polyphenol oxidases are enzymes that, in olive, oxidize o‐diphenols (principally oleuropein and rutin) to produce o‐diquinones and melanins, substances that are toxic to many pathogens. No significant increase in overall PPO activity was found in infected leaves; on the contrary, enzyme activity was gradually lost as infection progressed, most probably due to degradation of plastids within mesophyll cells, in which such enzymes are normally confined. Only a limited local PPO activation occurred in a few upper epidermal cells of the leaf, indicating a feeble induction of a plant response.     

Growth Complementation of hrpXo Mutants of Xanthomonas oryzae pv. oryzae by Virulent Strains in Rice Cultivars Resistant and Susceptible to the Parental Strain

Xanthomonas oryzae pv. oryzae (X. o. pv. oryzae) T7174 is virulent on rice cultivar IR24 and avirulent on IR-BB2. From recent reports, some virulence and avirulence factors of plant pathogenic bacteria are transferred to plant cells through the hrp-dependent type III secretion system. In this study, we investigated the involvement of hrp genes in the compatible and the incompatible interactions between rice and X. o. pv. oryzae after co-inoculation with hrpXo mutants derived from T7174 and virulent strains. Growth of the mutants, named 74ΔHrpXo and 76ΔHrpXo, was repressed in IR24 when the mutants were applied alone. However, growth of the mutants was complemented by co-inoculation with virulent strains. Growth of bioluminescent hrpXo mutant 76ΔHrpXo in IR24 and its growth in IR-BB2 after co-inoculation with T7133, which is virulent on both cultivars, was equally complemented, as detected by bioluminescence from the mutant. On the other hand, only partial complementation of growth of T7174L76, which is a bioluminescent and pathogenic derivative of T7174, by T7133 was observed in IR-BB2. Thus, growth of the hrpXo mutant of X. o. pv. oryzae was complemented by virulent strains in both susceptible and resistant rice leaves with the parental strain. Received 21 July 2000/ Accepted in revised form 26 October 2000     

Pseudomonas syringae pv. solidagae pv. nov., the Causal Agent of Bacterial Leaf Spot of Tall Goldenrod Solidago altissima L.

A new bacterial disease of tall goldenrod (Solidago altissima L., “Seitaka-awadachiso” in Japanese), one of the most serious weeds in non-agricultural land, was discovered in Ibaraki Prefecture, Japan. Characterized by angular or round, dark brown necrotic spots on leaves, this disease resulted in defoliation and terminal dieback of the plants in severe cases. The disease was named “bacterial leaf spot”. The causal bacterium was identified as Pseudomonas syringae based on its bacteriological properties including those determined by LOPAT tests. The present bacterium was pathogenic to tall goldenrod alone but not to many other tested plants including weeds, flowers, trees and crops. In addition, P. syringae pv. syringae and other pathovars did not show any pathogenicity to tall goldenrod. Because no pathovars of P. syringae pathogenic to tall goldenrod have been reported, the present bacterium was concluded to be a new pathovar of P. syringae. We propose the name P. syringae pv. solidagae pv. nov. , and strain Sei 1 (MAFF 810063) is designated as the pathotype strain and has been deposited in the MAFF collection with two reference strains (MAFF 810064 and MAFF81066). Received 9 May 2001/ Accepted in revised form 18 June 2001     

Evaluation of cucumber (<Emphasis Type="Italic">Cucumis sativus</Emphasis>) cultivars grown in Eastern Europe and progress in breeding for resistance to angular leaf spot (<Emphasis Type="Italic">Pseudomonas syringae</Emphasis> pv. <Emphasis Type="Italic">lachrymans</Emphasis>)

Increased occurrence of cucumber angular leaf spot, Pseudomonas syringae pv. lachrymans, has caused significant losses in cucumber, Cucumis sativus, yield in Poland in recent years. These losses necessitated evaluation of the level of resistance in cucumber cultivars of mainly Polish breeding, cultivated in Eastern Europe, and initiation of a breeding programme for resistance to this disease. Screening for resistance was performed on 84 cucumber accessions under growth chamber conditions using a highly aggressive strain of P. syringae pv. lachrymans. Most of the screened accessions were either susceptible or displayed intermediate resistance. The screening resulted in the identification of five F₁ hybrid cultivars moderately resistant to angular leaf spot. The identified F₁ hybrids were self-pollinated up to the F₄ generation. Individuals resistant to angular leaf spot were identified. These individuals can be used as a source of resistance to angular leaf spot in future breeding efforts.     

Note: Comparison of three laboratory methods to evaluate the pathogenicity and virulence of tenPseudomonas syringae pv.syringae strains on apple, pear, cherry and peach trees

The pathogenicity and virulence of ten GreekPseudomonas syringae pv.syringae strains from different hosts (citrus, pear, apple, peach and cherry) were evaluated using three different laboratory methods, which produced results in good agreement. All ten strains were virulent on apple, pear, cherry and peach trees. The extent of tissue colonized varied considerably among strains and cultivars. On excised shoots and twigs of apple and pear, strains BPI 176, BPI 203, PI 2 and PI 14 were the most virulent and strains BPI 689, BPI 992, BPI 4, BPI 20, PI 18 and PI 19 were the least virulent. On excised shoots and twigs of peach and cherry, strains BPI 176, BPI 203, PI 2, PI 14, PI 18 and PI 19 were the most virulent and strains BPI 4 and BPI 20 were the least virulent. Moderate virulence was evinced by strains BPI 689 and BPI 992. These pathogenicity assays are proposed as rapid and reproducible screening systems to evaluate the susceptibility of apple, pear, cherry and peach cultivars to this bacterial pathogen.     

Assessment of the response of carrot somaclones to Pythium violae, causal agent of cavity spot

Cavity spot is a major disease of carrots, causing cavities on the surface of the root. Available commercial varieties show a range of susceptibility but no significant resistance. Seed progeny from 46 tissue culture-derived carrot ( Daucus carota ) somaclones were screened for viability, then 19 selected somaclone families were sown under glasshouse conditions, along with commercial cultivars (Bertan, Nandor, Bolero and Vita Longa) as controls. Mature roots were exposed to Pythium violae in a cavity spot bioassay to determine their response as measured by disease incidence and severity. Some somaclones formed fewer lesions than the least susceptible control cultivar, Vita Longa. Seven somaclonal families that showed a range of susceptibility were sown under field conditions and the assessment was repeated. Although there was little relationship between glasshouse and field-trial results, under field conditions one of the somaclones had a mean incidence of disease, as estimated by transformed data, of 1·9 compared with 37·9 for the most susceptible somaclone and 3·5 for Bolero, the most resistant commercial cultivar. The results indicated that significant genetic variation in susceptibility to cavity spot disease was present in the somaclones.     

Movement of Xanthomonas campestris pv. vitians in the stems of lettuce and seed contamination

Xanthomonas campestris pv. vitians , the causal agent of bacterial leaf spot of lettuce (BLS), can be seedborne, but the mechanism by which the bacteria contaminates and/or infects lettuce seed is not known. In this study, the capacity of X. campestris pv. vitians to enter and translocate within the vascular system of lettuce plants was examined. The stems of 8- to 11-week-old lettuce plants were stab-inoculated, and movement of X. campestris pv. vitians was monitored at various intervals. At 4, 8, 12 and 16 h post-inoculation (hpi), X. campestris pv. vitians was recovered from 2 to 10 cm above (depending on stem length) and 2 cm below the inoculation site. Xanthomonas campestris pv. vitians was also recovered from surface-disinfested stem sections of spray-inoculated plants. Together, these results are consistent with X. campestris pv. vitians invading and moving systemically within the vascular system of lettuce plants. To investigate the mechanism of seed contamination, lettuce plants at the vegetative stage of growth were spray-inoculated with X. campestris pv. vitians and allowed to develop BLS. Seed collected from these plants had a 2% incidence of X. campestris pv. vitians external colonization, but no bacteria were recovered from within the seed.     

A comparison of methods to evaluate the susceptibility of potato cultivars to blackleg (caused by Erwinia carotovora subsp. atroseptica) in the field at different sites

Potato seed tubers were inoculated either using a needle or using a high-pressure jet injector at a number of sites in the United Kingdom. Over 2 years, different cultivars showed different trends in blackleg symptoms. Comparisons with inoculation by infiltration of tubers under vacuum were made at two sites, and by damaging followed by dipping tubers in inoculum at one site. The relationship between plant vigour in July and yield was closer than that between blackleg symptoms and yield.     

Effect of Soil Plowing and Fertilization on the Susceptibility of Four Olive Cultivars to the Insect<Emphasis Type="Italic">Bactrocera oleae</Emphasis> and the Fungi<Emphasis Type="Italic">Sphaeropsis dalmatica</Emphasis> and<Emphasis Type="Italic">Spilocaea oleagina</Emphasis>

Susceptibility to the insectBactrocera oleae and the fungiSpilocaea oleagina andSphaeropsis dalmatica was investigated in four olive cultivars, two for table fruit production (Kalamon and Chondrolia Chalkidikis) and two for oil production (Lianolia and Koroneiki). Cv. Chondrolia Chalkidikis was the most susceptible to all three pathogens, followed by cv. Kalamon. Soil plowing and the organic fertilizer Bio-Trust^® (10-3-6+8% MgCO₃+10% CaCO₈) increased the susceptibility of all four tested olive cultivars to the insect and the two fungi. Correlations were found between the attacks byB. oleae and infections byS. oleagina andS. dalmatica on the four olive cultivars.     

Physiological responses of Populus euphratica Oliv. to groundwater table variations in the lower reaches of Heihe River, Northwest China

Riparian vegetation in the lower reaches of Heihe River serves important ecological functions. However, the riparian ecosystems have been constantly deteriorating in the past 30 years simply due to water interception for oasis agricultural irrigation in the middle reaches of the river. This study pays a particular attention to Populus euphratica Oliv. forest because it is a dominant component of the riparian ecosystem in the lower reaches of Heihe River where the depth of groundwater table is the controlling factor in sustaining riparian ecosystems. To reveal leaf-related physiological responses of Populus euphratica Oliv. forest to groundwater table variations, we analyzed the relationships between the depth of groundwater table (DG) and three leaf-related parameters, i.e. leaf stomatal density (SD), specific leaf area (SLA), and stable carbon isotopic composition (δ 13 C). Our results show that the relationship between DG and leaf SD is a bi-mode one shaped by both salt stress and water stress. That is, salt stress appeared in shallow groundwater conditions and water stress happened in deep groundwater conditions, and the thin layer around 2.7 m of DG is a stress-free layer. Leaf SD fluctuated according to the DG variation, first decreased with increasing DG, then increased at depths ranging 2.7-3.7 m, and after a relatively stable plateau of SD at depths ranging 3.7-5.2 m, decreased again with increasing DG. Our results also show that SLA decreased exponentially with increasing DG and foliar δ 13 C values are also strongly dependent on DG, further demonstrating that these two parameters are sensitive indicators of water stress. The exponential curve suggests that SLA is more sensitive to DG when groundwater table is shallow and 3 m seems to be a threshold beyond which SLA becomes less sensitive to DG. Foliar δ 13 C becomes more sensitive when the groundwater table is deep and 7 m seems to be a threshold below which the δ 13 C signature becomes more sensitive to DG. These findings should be helpful in monitoring the growth and development of Populus euphratica Oliv. forests and also in providing protection measures (i.e. DG related) for Heihe River riparian forests.     

Enhanced resistance to common scab of potato through somatic cell selection in cv. Iwa with the phytotoxin thaxtomin A

A somatic cell selection approach, developed using thaxtomin A as a positive selection agent, recovered 113 potato cell colonies of cv. Iwa from which 39 regenerated plants within a single subculture period. In glasshouse pathogenicity trials, 13 of these regenerated variants exhibited significantly greater resistance to common scab disease than the unselected parent cultivar, both in mean tuber surface coverage with lesions and the proportion of disease free tubers. The best variant had on average an 85–86% lower disease score (or 91–92% less estimated tuber surface cover) than the unselected parent cultivar. The disease-resistant variants varied in their response to thaxtomin A in detached leaflet and tuber slice bioassays. This suggested that the cell selection procedure and thaxtomin A screening had limited success in selecting for toxin tolerance, and that thaxtomin A tolerance was not necessarily the driving factor for induction of disease resistance observed. Variation in relative necrosis scores between leaflet and tuber assays for individual variants possibly suggests differential expression of thaxtomin A tolerance in the different tissues. Yield estimates from the glasshouse grown potatoes showed the majority of disease-resistant variants had tuber yields equivalent to the unselected parent cultivar. This suggests that selection for disease resistance was not associated with negative tuber yield attributes and that these variants may have commercial merit and are worthy of further agronomic testing.