A theory relating focal epidemics to crop-weed interactions

The effects of dispersal gradient and pathogen life cycle components on epidemic velocity in computer simulations

ABSTRACT The velocity of expansion of focal epidemics was studied using an updated version of the simulation model EPIMUL, with model parameters relevant to wheat stripe rust. The modified power law, the exponential model, and Lambert's general model were fit to primary disease gradient data from an artificially initiated field epidemic of stripe rust and employed to describe dispersal in simulations. The exponential model, which fit the field data poorly (R (2) = 0.728 to 0.776), yielded an epidemic that expanded as a traveling wave (i.e., at a constant velocity), after an initial buildup period. Both the modified power law and the Lambert model fit the field data well (R(2) = 0.962 to 0.988) and resulted in dispersive epidemic waves (velocities increased over time for the entire course of the epidemic). The field epidemic also expanded as a dispersive wave. Using parameters based on the field epidemic and modified power law dispersal as a baseline, life cycle components of the pathogen (lesion growth rate, latent period, infectious period, and multiplication rate) and dispersal gradient steepness were varied within biologically reasonable ranges for this disease to test their effect on dispersive wave epidemics. All components but the infectious period had a strong influence on epidemic velocity, but none changed the general pattern of velocity increasing over time.

Genetic structure of the annual weed Senecio vulgaris in relation to habitat type and population size

Throughout the world, the highly selfing annual common groundsel, Senecio vulgaris (Asteraceae) is a common weed. Recently, it has also colonized ecological compensation areas in agro-ecosystems. We investigated the genetic structure of S. vulgaris using random amplified polymorphic DNA (RAPD) profiles of 80 plants from nine populations representing three habitat types in two regions in Switzerland. RAPD variation among regions (19.8%), among populations within regions (19.2%) and within populations (61.1%) was highly significant (ANOVA; P < 0.001). Gene flow estimated from the observed differentiation among populations (PhiST = 0.382) was low (assuming Wright's island model, Nem = 0.404). Genetic distances between pairs of populations were significantly correlated with geographical distances (Mantel test; r = 0.37, P < 0.03). Molecular variance obtained with AMOVA was lowest in the small populations in compensation areas (1.13), intermediate in vineyard populations (2.49), all located in northern Switzerland and highest in the larger vegetable field populations from western Switzerland (3.41; P < 0.05). Overall, there was a positive correlation of molecular variance and population size (P < 0.05), as expected under genetic drift. However, molecular variance was negatively correlated with population size among populations in ecological compensation areas, suggesting that selection was also important. We also applied triazine herbicide to leaves of three offspring of each of the 80 plants. Plants from populations of compensation areas showed higher mean levels and reduced variation in the resistance to triazine herbicide than plants from vineyards and vegetable fields. This suggests that compensation areas were colonized from adjacent corn fields, in which there has been selection for herbicide resistance. We discuss the implications of our results for the biological control of S. vulgaris.

Infection process and resistance in the weed pathosystem Senecio vulgaris - Puccinia lagenophorae and implications for biological control

The infection process and the level and type of resistance in the pathosystem of a native weed and a naturalized rust fungus, Senecio vulgaris L. - Puccinia lagenophorae Cooke, were examined. Four inbred plant lines from Switzerland (2), The Netherlands, and the United Kingdom, each at two stages of devolpment, were exposed to corresponding rust lines under controlled conditions. Fluorescence and light microscopy were used to assess the infection process and to quantify genotype effects. Component analysis was used to partition disease development and define the infection process. Germinating aeciospores of P. lagenophorae showed all the characteristics of the monokaryotic parasitic stage of rust infection with direct penetration and monokaryotic haustoria formation. Haustoria formed between 3 and 6 days after inoculation at low frequency. The highest level of resistance, for which differences between plant lines were detected, occurred at penetration peg formation. All the studied host-pathogen interactions were compatible. The Dutch plant line was most susceptible to all rust lines, and the Dutch rust line was the most aggressive. Susceptibility of leaves increased with the leaf developmental stage. There was a continuous range of variation in susceptibility without differential genetic interactions, thus indicating race-nonspecific quantitative resistance. The use of the more aggressive Dutch rust line may increase the level of disease, thus stimulating epidemics for biological control. Over the long term, less susceptible genotypes of the weed may be selected, although differences in susceptibility among plant lines were relatively small.Key words: rust fungus, infection structures, direct penetration, component analysis, race-nonspecific quantitative resistance.