Library

Notes: To obtain a better understanding of the onset and the progress of epidemics caused by Diaporthe adunca on Plantago lanceolata we investigated the role of the asexual phase of the fungus not only in the transmission of the disease from season to season, but also in the spread of the fungus within the same season. Splash dispersal of conidia during rain episodes proved to be an effective method for the spread of the fungus. Conidial production by pycnidia present on scapes left on the ground during winter was delayed in comparison to that of pycnidia produced on greenhouse infected scapes, indicating that the fungus overwinters with immature pycnidia. Latent infections occurred in 8.3% of the seedlings which emerged from seeds originating from infected spikes. A scenario for the transmission and spread of the fungus is proposed.

Notes: 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.

Notes: Radial expansion of foci in mixtures of susceptible and resistant bean cultivars was studied at two sites in Ethiopia. The foci expanded in a wave-like fashion. At Ambo (1990), radial expansion velocity ranged from 6 cm per day in mixtures with 20% susceptible plants to 15 cm per day in plots with the susceptible plants only. At Debre Zeit, the velocity ranged from 3 cm per day in a mixture with 20% susceptible plants to 16 cm per day in plots with 100% susceptible plants. At both sites the radial expansion velocity of foci correlated linearly with the logarithm of the fraction of susceptible plants in the mixture. Velocities of focus expansion at Ambo and Debre Zeit were approximately equal in plots consisting of susceptible plants only. At lower proportions of susceptible plants the velocities at Debre Zeit were lower than at Ambo. Indications were given as to the environmental factors responsible for the observed difference between sites. At each site, the variation between plots showed a clear spatial pattern, probably due to environmental factors.

Notes: Epidemics of faba bean rust (Uromyces viciae-fabae) differing in duration and final severity were induced by a point inoculation. Relationships between disease severity observed on three canopy layers (top, middle and bottom) at different assessment dates and yield components (grain weight per stem, average grain weight, number of grains per pod and number of pods per stem) were established. Epidemics occurring after the initiation of pod setting had no effect on yield components. Earlier epidemics significantly decreased the grain weight per stem, the average grain weight, and the number of pods per stem, but only marginally affected the number of grains per pod. The yield components generally decreased with increasing severity level, but a few parabolic relationships were observed at a late stage in crop development. The effect of rust on yield components could be predicted by a critical point model using disease severity assessed on the middle or bottom canopy layer in the mid-flowering stage. Rust severity greater than 5% during that critical stage would substantially decrease final yield, and fungicide spraying against rust would be advised.

Notes: The dispersal of conidia of Mycocentrospora acerina was studied in caraway field trials. A Burkard spore trap, rotorods, inverted Petri dishes containing sucrose agar and rain gauges were used to trap conidia of M. acerina. Sporulation was stimulated by rainfall (〈inlineGraphic alt="geqslant R: gt-or-equal, slanted" extraInfo="nonStandardEntity" href="urn:x-wiley:00320862:PPA143:ges" location="ges.gif"/〉2 mm) and moderate temperatures (around 15°C). Solar radiation had a negative effect on sporulation. Hardly any conidia were found in the spore traps on rainless days. Short distance (〈inlineGraphic alt="leqslant R: less-than-or-eq, slant" extraInfo="nonStandardEntity" href="urn:x-wiley:00320862:PPA143:les" location="les.gif"/〉9 m) spread of M. acerina is mainly caused by splash dispersal of its conidia. Trap plants at 0, 0.1, 1 and 4 m from the inoculum source were readily infected under moist conditions. Beyond 9 m from an inoculum source no infection of caraway trap plants was found. Trap plants at 9 m from an inoculum source were infected in one out of three seasons only. Long distance (〉9 m) spread could not be demonstrated by the techniques used in this study. The results suggest that, usually, a caraway field is infected by inoculum sources within that field.

Notes: Epidemics of the splash-dispersed pathogenic fungus Diaporthe adunca on its host, the perennial herb Plantago lanceolata, were followed during two consecutive years in transects at roadsides in the Netherlands. Epidemics of D. adunca were also studied on clones of a susceptible and a partially resistant genotype of P. lanceolata grown either in a pure stand or in a 1:1 mixture in small plots in the garden. The epidemics in the natural and experimental populations could be adequately described by logistic and Gompertz models, but large differences were found in final disease levels and relative growth rates. The effect of partial resistance on the epidemic in the mixture was less than in a pure stand, probably due to the provision of inoculum from the highly diseased susceptible genotype to the partially resistant genotype. In the garden focal and wind-direction effects were seen. In the natural populations the epidemics developed from numerous primary infected scapes making foci and wind-directions effects less conspicuous.

Notes: Abstract Spatio-temporal progress of an epidemic of faba bean rust was monitored over a discontinuous field. Trap plots were sown at increasing distances from a source plot, in the centre of which plants were inoculated. Disease spread in the source plot followed a focal pattern, with a radial velocity of expansion slightly lower than 0.1 m per day. At the end of the experiment, all trap plots had been infected, and two of the most distant ones showed unexpected high disease severity. Using a three-dimension model of disease progress, we showed that the epidemics on the scales of the source plot and of the trap plots could not be combined into a single epidemic on the whole-field scale. The epidemics had equivalent infection rates on both scales, but changing the scale dramatically affected their distance parameter. The epidemic in the source plot could have been caused by a short-distance, high-frequency, deterministic mechanism of spore dispersal, whereas infection of the trap plots could have been caused by a long-distance, low-frequency stochastic mechanism of spore dispersal. Although our results agreed with the predictions of a simulation model postulating these two mechanisms, alternative hypotheses which could also explain the observed disease pattern remained to be tested.

Notes: Summary Horizontal, uniform, race-non-specific or stable resistance can be discerned according to Van der Plank, from vertical, differential, race-specific or unstable resistance by a test in which a number of host genotypes (cultivars or clones) are tested against a number of pathogen genetypes traces of isolatest. If the total non-environmental variance in levels of resistance is due to main effects only differences between cultivars and differences between isolates) the resistance and the pathogen many (in the broad sense) are horizontal in nature. Vertical resistance and pathogenicity are characterized by the interaction between host and pathogen showing up as a variance compenent in this test due to interaction between cultivars and isolates. A host and pathogen model was made in which resistance and pathogenicity are governed by live polygenic loci. Within the host the resistance genes show additivity. Two models were investigated in model I resistance and pathogenicity genes operate in an additive way as envisaged by Van der Plank in his horizontal resistance. Model II is characterized by a gene-for-gene action between the polygenes of the host and those of the pathogen. The cultivar isolate test in model I showed only main effect variance. Surprisingly, the variance in model II was also largely due to main effects. The contribution of the interaction to the variance uppeared so small, that it would be difficult to discern it from a normal error variance. So-called horizontal resistance can therefore be explained by a polygenic resistance, where the individual genes are vertical and operating on a gene-for-gene basis with virulence genes in the pathogen. The data reported so far support the idea that model II rather than model I is the realistic one. The two models also revealed that populations with a polygenic resistance based on the gene-for-gene action have an increased level of resistance compared with the addition model, while its stability as far as mutability of the pathogen is concerned, is higher compared to those with an additive gene action. Mathematical studies of Mode too support the gene-for-gene concept. The operation of all resistance and virulence genes in a natural population is therefore seen as one integrated system. All genes for true resistance in the host population, whether they are major or minor genes are considered to interact in a gene-for-gene way with virulence genes either major or minor, in the pathogen population. The models revealed other important aspects. Populations with a polygenic resistance based on a gene-for-gene action have an increased level of resistance compared to populations following the addition model. The stability, as far as mutability of the pathogen is concerned, is higher in the interaction model than in the addition model. The effect of a resistance gene on the level of resistance of the population consists of its effect on a single plant times its gene frequency in the population. Due to the adaptive forces in both the host and the pathogen population and the gene-for-gene nature of the gene action an equilibrium develops that allows all resistance genes to remain effective although their corresponding virulence genes are present. The frequencies of the resistance and virulence genes are such that the effective frequencies of resistance genes tend to be negatively related to the magnitude of the gene effect. This explains why major genes often occur at low frequencies, while minor genes appear to be frequent. It is in this way that the host and the pathogen, both as extremely variable and vigorous populations, can co-exist. Horizontal and vertical resistance as meant by Van der Plank therefore do not represent different kinds of resistances, they represent merely polygenic and oligogenic resistances resp. In both situations the individual host genes interact specifically with virulence genes in the pathogen. Van der Plank's test for horizontal resistance appears to be a simple and sound way to test for polygenic inheritance of resistance. The practical considerations have been discussed. The agro-ecosystems should be made as diverse as possible. Multilines, polygenic resistance, tolerance, gene deployment and other measures should be employed, if possible in combination.

Notes: Abstract Phaseolus bean cultivars, obtained from the Ethiopian national breeding programme, and cultivars widely grown in the country, 15 in total, were tested in a greenhouse for five components of partial resistance to one isolate of bean rust. The single-pustule isolate came from Ambo, a site where bean lines are tested against rust because the climate is conducive to bean rust. The components examined include latent period (LP50), infection efficiency (IE), sporulation capacity (SC), infectious period (IP) and pustule size (PS). Differences in cultivar responses were found for all PR components. Differences were largest, however, for infection efficiency and sporulation capacity. Cultivars Exrico 23, A 176, Veracruz 10 and BAT 1198 had a high level of PR to the isolate tested. Mexican 142, a widely grown cultivar in Ethiopia was intermediate, whereas Red Wolaita, an important cultivar in the south, showed a low level of PR. Linear correlations between LP50 and IE, and between SC and PS were high. Linear correlations between IE, SC, or PS with IP were not significant. Though differences in cultivar response were found for all components, any one parameter may not suffice to explain the PR potential of a particular cultivar. The study suggests that latent period, infection efficiency and pustule size should be used in selection for PR. For the evaluation of large numbers of bean cultivars in the greenhouse, IE and PS are preferable to minimize labour requirements.