M.J. Jeger

A Theoretical Assessment of the Effects of Vector-Virus Transmission Mechanism on Plant Virus Disease Epidemics

ABSTRACT A continuous-time and deterministic model was used to characterize plant virus disease epidemics in relation to virus transmission mechanism and population dynamics of the insect vectors. The model can be written as a set of linked differential equations for healthy (virus-free), latently infected, infectious, and removed (postinfectious) plant categories, and virus-free, latent, and infective insects, with parameters based on the transmission classes, vector population dynamics, immigration/emigration rates, and virus-plant interactions. The rate of change in diseased plants is a function of the density of infective insects, the number of plants visited per time, and the probability of transmitting the virus per plant visit. The rate of change in infective insects is a function of the density of infectious plants, the number of plants visited per time by an insect, and the probability of acquiring the virus per plant visit. Numerical solutions of the differential equations were used to determine transitional and steady-state levels of disease incidence (d*); d* was also determined directly from the model parameters. Clear differences were found in disease development among the four transmission classes: nonpersistently transmitted (stylet-borne [NP]); semipersistently transmitted (foregut-borne [SP]); circulative, persistently transmitted (CP); and propagative, persistently transmitted (PP), with the highest disease incidence (d) for the SP and CP classes relative to the others, especially at low insect density when there was no insect migration or when the vector status of emigrating insects was the same as that of immigrating ones. The PP and CP viruses were most affected by changes in vector longevity, rates of acquisition, and inoculation of the virus by vectors, whereas the PP viruses were least affected by changes in insect mobility. When vector migration was explicitly considered, results depended on the fraction of infective insects in the immigration pool and the fraction of dying and emigrating vectors replaced by immigrants. The PP and CP viruses were most sensitive to changes in these factors. Based on model parameters, the basic reproductive number (R(0))-number of new infected plants resulting, from an infected plant introduced into a susceptible plant population-was derived for some circumstances and used to determine the steady-state level of disease incidence and an approximate exponential rate of disease increase early in the epidemic. Results can be used to evaluate disease management strategies.

Epidemic dynamics and patterns of plant diseases

ABSTRACT The general Kermack and McKendrick epidemic model (K&M) is derived with an appropriate terminology for plant diseases. The epidemic dynamics and patterns of special cases of the K&M model, such as the Vanderplank differential-delay equation; the compartmental healthy (H), latent (L), infectious (S), and postinfectious (R) model; and the K&M model with a delay-gamma-distributed sporulation curve were compared. The characteristics of the disease cycle are summarized by the basic reproductive number, R(0), and the normalized sporulation curve, i(tau). We show how R(0) and the normalized sporulation curve can be calculated from data in the literature. There are equivalences in the values of the basic reproductive number, R(0), the epidemic threshold, and the final disease level across the different models.However, they differ in expressions for the initial disease rate, r, and the initial infection, Q, because the values depend on the sporulation curve. Expressions for r and Q were obtained for each model and can be used to approximate the epidemic curve by the logistic equation.

Disease control and its selection for damaging plant virus strains in vegetatively propagated staple food crops; a theoretical assessment.

Viral diseases are a key constraint in the production of staple food crops in lesser developed countries. New and improved disease control methods are developed and implemented without consideration of the selective pressure they impose on the virus. In this paper, we analyse the evolution of within-plant virus titre as a response to the implementation of a range of disease control methods. We show that the development of new and improved disease control methods for viral diseases of vegetatively propagated staple food crops ought to take the evolutionary responses of the virus into consideration. Not doing so leads to a risk of failure, which can result in considerable economic losses and increased poverty. Specifically in vitro propagation, diagnostics and breeding methods carry a risk of failure due to the selection for virus strains that build up a high within-plant virus titre. For vegetatively propagated crops, sanitation by roguing has a low risk of failure owing to its combination of selecting for low virus titre strains as well as increasing healthy crop density.

Yield Loss in Apple Caused by Monilinia fructigena (Aderh. & Ruhl.) Honey, and Spatio-temporal Dynamics of Disease Development

Monilinia fructigena (Aderh. & Ruhl.) Honey causes considerable yield losses in pome fruit culture. During a field study in the Netherlands in 1997 and 1998, the increase in disease incidence in time was assessed and final pre- and post-harvest losses were recorded in the susceptible apple cultivars James Grieve and Coxs Orange Pippin. Each individual tree was considered as a unique quadrat, and the spatial distribution of diseased fruits among fruit trees at every assessment date was characterised by a dispersion index, Lloyds index of patchiness (LIP). Spatial autocorrelation was applied to detect potential clustering of trees with diseased fruits within rows. In cv. James Grieve, the rate of increase of disease incidence was constant up to harvest time, whereas in cv. Coxs Orange Pippin disease incidence increased markedly 3 weeks before harvest time, which coincided with the harvest of cv. James Grieve in neighbouring rows. Pre-harvest disease incidence was 4.2–4.3% in cv. James Grieve in both years, in cv. Coxs Orange Pippin this was 4.4% in 1997 and 2.7% in 1998. Post-harvest yield losses amounted on average 1.5–2.0% for both cultivars, no significant differences were found between the cultivars (t-test, P=0.05). Both in 1997 and 1998, clustering of diseased fruits among fruit trees was detected; LIP values were significantly higher than 1 (P=0.05 in 1997, P=0.01 in 1998). Clustering of trees with diseased fruits was detected in 1998, when significant (P=0.05) positive correlation coefficients occurred for 2nd, 3rd and 4th lag-order distances in cv. James Grieve, and a significant (P=0.05) positive first-order correlation in cv. Coxs Orange Pippin. Wounding agents, such as insects and birds, may play an important role in the underlying disease dynamics, and crop losses may be minimised by control of these agents.

Epidemiology in Relation to Methods for Forecasting Light Leaf Spot (Pyrenopeziza brassicae) Severity on Winter Oilseed Rape (Brassica napus) in the UK

Pyrenopeziza brassicae, cause of light leaf spot of oilseed rape, has a complex polycyclic life cycle. It can be difficult to control light leaf spot in winter oilseed rape in the UK since it is not easy to optimise fungicide application timing. Early autumn infections are usually symptomless and recognisable lesions do not develop until the epidemic has progressed further by the spring. Light leaf spot often has a patchy distribution in winter oilseed rape crops and estimation of disease incidence can be difficult. There is evidence that epidemics are initiated primarily by ascospores produced from apothecia that survive the summer inter-crop period on infected debris. Subsequent development of the epidemic during the winter and spring is maintained by rain-splashed conidia that spread light leaf spot from initial foci. Understanding the relative roles of ascospores and conidia in the light leaf spot life cycle is crucial for forecasting epidemic severity and developing control strategies. The current web-based regional forecast system provides an autumn forecast of the incidence of light leaf spot that can be expected the following spring. This is based on survey data which assesses the occurrence of disease the previous July, and weather factors, such as deviations from summer mean temperature and winter rainfall. The forecast can be updated throughout the autumn and winter and includes crop-specific elements so that growers can adjust risks by inputting information about cultivar, sowing date and fungicide use. Crop-specific forecasts can be confirmed by assessing the incidence of light leaf spot. Such assessments will become easier when immunodiagnostic methods for detection of the disease become available. Incorporation of information on spore biology (e.g. apothecial maturation, ascospore release and infection conditions) is considered as a component of the interactive, continuously updated, crop-specific, web-based forecasts which are needed in the future.

Distinction of the Asiatic brown rot fungus Monilia polystroma sp. nov. from M. fructigena

Monilinia fructigena isolates from Japan were compared with isolates from Europe. General colony characteristics, stroma formation, growth rate and conidial dimensions were determined for six isolates each from both groups, as well as sporulation intensity on potato dextrose agar (PDA) and germ tube features. Potential differences in pathogenicity were tested on the pear cultivars ‘Conference’ and ‘Doyenne du Comice’, and on the apple cultivar ‘Coxs Orange Pippin’. A marked difference in stroma formation occurred, the area of stromatal plates ranged from 4.1 to 5.2 cm 2 in the Japanese group, and from 0 to 0.9 cm 2 in the European. The mean growth rate was significantly higher for Japanese isolates ( t -test, P = 0.01). Length and width of conidia were significantly greater in European isolates ( t -test, P = 0.01). Conidia measured on average 19 x 11.5 μm in European isolates, and 16 x 10 μm in Japanese ones when grown on cherry agar. On fruits, the difference in conidium size was even more pronounced. Sporulation intensity on PDA and germ tube features did not differ between both groups. No differences were found in latency period, lesion growth rate or sporulation intensity on apple and pear fruits between both groups. Together with previously published differences in the ITS region of ribosomal DNA, our results show that the Japanese isolates belong to a distinct species, Monilia polystroma sp. nov. A description of the anamorph is given, as well as a table summarising key features for all four brown rot associated Monilia species.

Space-time statistics for environmental and agricultural related phenomena

This paper presents an overview of space-time statistical procedures to analyse agricultural and environmental related phenomena. It starts with an application on root-rot development in cotton. Dependence modelling in space and time is done with the space-time variogram. Various kriging interpolators are presented for making predictions in space and time. Simulated annealing is used to design an optimal monitoring network for estimation of space-time variograms. In the application no clear indication was found for anisotropy, although strong evidence exists that the disease not only proceeds within rows but also jumps between rows. The optimal sampling scheme showed a spatial clustering of observations at the first and the last monitoring day and less observations at intermediate times.

Simulation of vertical spread of plant diseases in a crop canopy by stem extension and splash dispersal

Upward displacement of light leaf spot lesions by winter oilseed rape stem extension and dispersal of Pyrenopeziza brassicae conidia by rain-splash were both incorporated into a model to describe vertical spread of light leaf spot on winter oilseed rape in the spring. Development of leaves, flowers and pods over the period from the start of stem extension to pod ripening was simulated. In the model, as new plant parts developed from the apex during stem extension, infections on plant initials were spread to the upper canopy by internode growth. In addition, conidia produced by the pathogen were dispersed in the canopy by rain-splash and produced infections at new sites. Vertical disease spread was simulated for a number of different light leaf spot distributions at the start of stem extension and with different crop structures and rain durations. Results showed that stem extension was an important factor in influencing vertical light leaf spot spread in the model oilseed rape crop. Rain events contributed to the splash dispersal of conidia to the plant apex and the resulting infections were directed vertically by internode growth. Periods with frequent rain events in a dense crop canopy (LAI constant with height) were most favorable for light leaf spot progress. The upward spread of light leaf spot on winter oilseed rape in field experiments showed the same trends as those predicted by the model.

Effects of Environmental Factors on Development of Pyrenopeziza brassicae (Light Leaf Spot) Apothecia on Oilseed Rape Debris

ABSTRACT The development of Pyrenopeziza brassicae (light leaf spot) apothecia was studied on petiole debris from artificially infected oilseed rape leaves incubated at temperatures from 6 to 22 degrees C under different wetness regimes and in 16 h light/8 h dark or continuous darkness. There was no significant difference between light treatments in numbers of apothecia that developed. Mature apothecia developed at temperatures from 5 to 18 degrees C but not at 22 degrees C. The rate of apothecial development decreased as temperature decreased from 18 to 5 degrees C; mature apothecia were first observed after 5 days at 18 degrees C and after 15 days at 6 degrees C. Models were fitted to estimates of the time (days) for 50% of the maximum number of apothecia to develop (t(1); model 1, t(1) = 7.6 + 55.8(0.839)(T)) and the time for 50% of the maximum number of apothecia to decay (t(2); model 2, t(2) = 24.2 + 387(0.730)(T)) at temperatures (T) from 6 to 18 degrees C. An interruption in wetness of the petiole debris for 4 days after 4, 7, or 10 days of wetness delayed the time to observation of the first mature apothecia for approximately 4 days and decreased the number of apothecia produced (by comparison with continuous wetness). A relationship was found between water content of pod debris and electrical resistance measured by a debris-wetness sensor. The differences between values of t(1) predicted by model 1 and observed values of t(1) were 1 to 9 days. Model 2 did not predict t(2); apothecia decayed more quickly under natural conditions than predicted by model 2.

Modeling the Dynamics of a Fungal Mycoparasite and Its Host: An Energy-Based Approach

First, third, and fourth authors: Department of Phytopathology, Wageningen Agricultural University, P.O. Box 8025, 6700 EE Wageningen, the Netherlands; second author: Department of Mathematics, Wageningen Agricultural University, Dreijenlaan 4, 6703 HA Wageningen, the Netherlands. Current address of C. Stolk: Department of Entomology, Wageningen Agricultural University, P.O. Box 8031, 6700 EH Wageningen, the Netherlands. Accepted for publication 13 February 1998.