Effect of crop growth and canopy filtration on the dynamics of plant disease epidemics spread by aerially dispersed spores

Most mathematical models of plant disease epidemics ignore the growth and phenology of the host crop. Unfortunately, reports of disease development are often not accompanied by a simultaneous and commensurate evaluation of crop development. However, the time scale for increases in the leaf area of field crops is comparable to the time scale of epidemics. This simultaneous development of host and pathogen has many ramifications on the resulting plant disease epidemic. First, there is a simple dilution effect resulting from the introduction of new healthy leaf area with time. Often, measurements of disease levels are made pro rata (per unit of host leaf area or total root length or mass). Thus, host growth will reduce the apparent infection rate. A second, related effect, has to do with the so-called "correction factor," which accounts for inoculum falling on already infected tissue. This factor accounts for multiple infection and is given by the fraction of the host tissue that is susceptible to disease. As an epidemic develops, less and less tissue is open to infection and the initial exponential growth slows. Crop growth delays the impact of this limiting effect and, therefore, tends to increase the rate of disease progress. A third and often neglected effect arises when an increase in the density of susceptible host tissue results in a corresponding increase in the basic reproduction ratio, R(0), defined as the ratio of the total number of daughter lesions produced to the number of original mother lesions. This occurs when the transport efficiency of inoculum from infected to susceptible host is strongly dependent on the spatial density of plant tissue. Thus, crop growth may have a major impact on the development of plant disease epidemics occurring during the vegetative phase of crop growth. The effects that these crop growth-related factors have on plant disease epidemics spread by airborne spores are evaluated using mathematical models and their importance is discussed. In particular, plant disease epidemics initiated by the introduction of inoculum during this stage of development are shown to be relatively insensitive to the time at which inoculum is introduced.     

Current knowledge on plant/canopy architectural traits that reduce the expression and development of epidemics

To reduce the use of pesticides, innovative studies have been developed to introduce the plant as the centre of the crop protection system. The aim of this paper is to explain how architectural traits of plants and canopies induce a more or less severe epidemic and how they may be modified in order to reduce disease development. In particular, it focuses on three key questions: i) which processes linked to epidemics can be influenced by architecture ii) how can architecture be characterized relative to these modes of action, and iii) how can these effects be explored and exploited? The roles of plant/canopy architecture on inoculum interception, on epidemic development via the microclimate and on tissue receptivity are discussed. In addition, the concepts of disease avoidance, canopy porosity and an ideotype unfavourable for disease development are described. This paper shows that many advances have already been made, but progress is still required in four main fields: microclimatology, mathematical modelling of plants, molecular genetics and ideotype conception.     

The effect of powdery mildew (Erysiphe graminis f. sp. hordei) and leaf rust (Puccinia hordei) on spring barley in New Zealand. II. Apical development and yield potential

Reduced yields caused by powdery mildew and leaf rust in two seasons were associated with reduced plant growth. Combinations of early, late and full epidemics in one season, and 12 epidemic combinations in the second, were designed to identify crop sensitivity to disease by comparing growth and development with healthy plants. Early epidemics reduced ear number by increasing tiller death, and reduced grain number by effects on spikelet, floret or grain abortion, depending on the type of epidemic. Epidemics later in crop growth increased floret and grain abortion and also reduced grain weight.
There was no compensation by later-determined components for reduced growth and delayed development at earlier growth stages. Plants infected at early growth stages were more sensitive to late infections, seen as effects on the later-determined components, than plants which were healthy initially. Interactions occurred between epidemics at different times and are likely to occur between diseases and other constraints.     

Soil tillage and eyespot: influence of crop residue distribution on disease development and infection cycles

Two deep-working soil tillage tools, one which inverts soil (plough) and one which does not (chisel), were used before sowing wheat after various crop successions combining eyespot host and non-host crops. Soil structure was nearly the same and crop residues were located in the different soil layers. Eyespot sporulation was estimated by visually assessing pot plants which had been on the trial plots for a fixed length of time. Field plants were also assessed for disease at several wheat growth stages. A kinetic equation expressing disease level as a function of degree-days was fitted to the disease levels observed on the field plants. This equation is based on eyespot epidemiology and depends on two parameters reflecting the importance of the primary and the secondary infection cycles respectively. Pot plant and early field plant disease levels and primary infection were closely correlated to the presence of crop residues in the top layer. The amount of residues depended on both crop succession and soil tillage. Where the previous crop was a host crop preceded by a non-host crop, soil inversion buried host residues, thus decreasing the primary infection risk. Where however the previous crop was a non-host crop preceded by a host crop, soil inversion carried the host residues back to soil surface, thus increasing the primary infection risk. Secondary infection was not correlated to either crop succession or soil tillage.     

Cropping systems modify soil biota effects on wheat (Triticum aestivum) growth and competitive ability

Plants alter soil biota which subsequently modifies plant growth, plant–plant interactions and plant community dynamics. While much research has been conducted on the magnitude and importance of soil biota effects (SBEs) in natural systems, little is known in agro‐ecosystems. We investigated whether agricultural management systems could affect SBEs impacts on crop growth and crop–weed competition. Utilising soil collected from eight paired farms, we evaluated the extent to which SBEs differed between conventional and organic farming systems. Soils were conditioned by growing two common annual weeds: Amaranthus retroflexus (redroot pigweed) or Avena fatua (wild oat). Soil biota effects were measured in wheat (Triticum aestivum) growth and crop–weed competition, with SBEs calculated as the natural log of plant biomass in pots inoculated with living soil divided by the plant biomass in pots inoculated with sterilised soil. SBEs were generally more positive when soil inoculum was collected from organic farms compared with conventional farms, suggesting that cropping systems modify the relative abundance of mutualistic and pathogenic organisms responsible for the observed SBEs. Also, as feedbacks became more positive, crop–weed competition decreased and facilitation increased. In annual cropping systems, SBEs can alter plant growth and crop–weed competition. By identifying the management practices that promote positive SBEs, producers can minimise the impacts of crop–weed competition and decrease their reliance on off‐farm chemical and mechanical inputs to control weeds, enhancing agroecosystem sustainability.     

Influence of crop management on eyespot development and infection cycles of winter wheat

Wheat was assessed at four crop growth stages for eyespot (anamorph Pseudocercosporella herpotrichoides, teleomorph Tapesia yallundae) in a series of field trials that studied the effects on disease frequency of five wheat management techniques (sowing date and density, nitrogen fertiliser dose and form, removal/burial of cereal straw). An equation expressing disease level as a function of degree-days was fitted to the observed disease levels. This equation was based on eyespot epidemiology and depended on two parameters illustrating the importance of the primary and the secondary infection cycles respectively. Cultural practices were classified according to the importance of their effects on disease, and these effects could be related to infection cycles and host plant architecture. Sowing date had the earliest and strongest effect; early sowing always increased disease frequency through the primary infection cycle, and its influence on the secondary cycle was variable. Disease frequency was increased by high plant density and/or a low shoot number per plant through primary infection; the secondary cycle was, however, decreased by a low shoot number per plant, which reduced late disease development at high plant density. High nitrogen doses increased disease levels and the severity of both infection cycles, but this effect was partly hidden by a simultaneous stimulation of tillering and thus an indirect decrease of disease incidence. When significant, ammonium (vs ammonium nitrate) fertiliser decreased eyespot levels and infection cycles whereas straw treatment (burial vs removal of straw from the previous cereal crop) had no effect.     

植物病害时空流行动态模拟模型的构建

 一个描述在二维空间中单一种植或混合种植的植物群体内病害时、空流行动态的计算机随机模拟模型构建完成。模型由寄主、病原2个组分和病斑产孢、孢子传播、孢子着落、孢子侵染、病斑潜育、寄主生长、病害控制等一系列代表病害流行生物学过程的子模型构成。模型采用了面向对象的程序设计方法,用C++语言编写,能以病害流行曲线图、空间分布图、数据列表等方式显示模拟结果。测试结果表明:模型能反映植物病害流行过程的本质规律,既可作为植物病害流行学教学工具,帮助学生理解病害流行的时、空动态规律和不同因子对病害流行的影响,也可以作为研究工具,对流行学的某些理论问题进行模拟研究     

Residues of pesticide co-formulants in lettuce and parsley: Identification of decline processes using field trials in different cropping systems

BACKGROUND

Although co-formulants constitute a substantial portion of the total plant protection product (PPP) mass applied to crops, data on residue formation and the behaviour of these substances on plants are scarce. In an earlier study we demonstrated that co-formulants commonly used in PPPs can form considerable residues, i.e., in the low to medium mg/kg range, but normally decline rapidly within few days. In the field trial reported here, we aimed to identify the major decline processes of co-formulants. Residues of co-formulants were therefore monitored in parsley and lettuce grown in an open field as well as under foil tunnels equipped with either an overhead or a drip irrigation system.

RESULTS

Dissipation of three anionic surfactants was markedly faster when crops (parsley and lettuce) were exposed to natural rainfall or irrigation from above compared to drip irrigation. In contrast, the decline of three volatile organic solvents was not affected by rain or irrigation, but was dependent on the crop, with much shorter half-lives in lettuce than in parsley. Furthermore, dilution through plant growth contributed significantly to the reduction of residues over time.

CONCLUSION

In this work we substantiate earlier findings on the magnitude and dissipation of residues of anionic surfactants and solvents representing the most important co-formulant classes. The chosen experimental setup allowed differentiation between decline processes and we confirm that foliar wash-off is a major dissipation process for anionic surfactants. For volatile organic solvents, dissipation appears to depend on the properties not only of the substance but also of the plant (surface). © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.     

Climate change impacts on plant canopy architecture: implications for pest and pathogen management

Climate change influences on pests and pathogens are mainly plant-mediated. Rising carbon dioxide and temperature and altered precipitation modifies plant growth and development with concomitant changes in canopy architecture, size, density, microclimate and the quantity of susceptible tissue. The modified host physiology and canopy microclimate at elevated carbon dioxide influences production, dispersal and survival of pathogen inoculum and feeding behaviour of insect pests. Elevated temperature accelerates plant growth and developmental rates to modify canopy architecture and pest and pathogen development. Altered precipitation affects canopy architecture through either drought or flooding stress with corresponding effects on pests and pathogens. But canopy-level interactions are largely ignored in epidemiology models used to project climate change impacts. Nevertheless, models based on rules of plant morphogenesis have been used to explore pest and pathogen dynamics and their trophic interactions under elevated carbon dioxide. The prospect of modifying canopy architecture for pest and disease management has also been raised. We offer a conceptual framework incorporating canopy characteristics in the traditional disease triangle concept to advance understanding of host-pathogen-environment interactions and explore how climate change may influence these interactions. From a review of recent literature we summarize interrelationships between canopy architecture of cultivated crops, pest and pathogen biology and climate change under four areas of research: (a) relationships between canopy architecture, microclimate and host-pathogen interaction; (b) effect of climate change related variables on canopy architecture; (c) development of pests and pathogens in modified canopy under climate change; and (d) pests and pathogen management under climate change.     

Biologically active substances from higher plants: Status and future potential

Plant breeding and selection, husbandry techniques and crop protection technology, including agrochemicals, have all made substantial contributions to the present-day level of crop productivity. However, yield losses due to disease, pests and weeds must continue to be minimized in order to meet the food supply demands from an ever growing population. Appropriate synthetic chemicals are becoming increasingly difficult to discover and develop due to stricter requirements on efficacy, selectivity, toxicology and general environmental impact. Consequently, there is a growing interest in understanding and utilising natural mechanisms as the basis for crop protection products. Plants themselves are a rich source of biologically active substances which could potentially be harnessed to modify crop growth or to protect crops against disease and pests. This review describes briefly the current status of understanding relative to plant–plant (herbicide and plant growth substances), plant–fungal (fungicide) and plant–insect (insecticide) interactions. Future prospects are considered in relation to directed synthesis, cell culture, microbial pesticides and plant genetic engineering. The opinion expressed is optimistic and suggests that science today can be utilised to secure the food supply of tomorrow. However, utilising either natural products or molecular biology may require an improved understanding of crop physiology and new developments in agronomy. Therefore, the time-frame for major impact of the ‘new’ technologies on crop productivity may be longer than is commonly predicted.     

The Conductance model of plant growth and competition in monoculture and species mixtures: a review

Over the past two decades, an ecophysiological model has been developed for annual horticultural crops and weeds, which has the powerful ability to predict the growth of plants in monoculture and mixed species stands from parameter values derived from plants grown in isolation, even if the species display contrasting canopy architecture. The model can also simulate the effects of different spatial arrangements on plant growth. The purpose of the model is to describe, in simple yet mechanistically‐based terms, the effects of contrasting environments and competitive interactions on the growth of individual plants. In the simplest form of the model, growth is described by an empirical growth equation, using time calculated from an integration of the growth‐promoting effects of environmental factors. More complex versions of the model include a self‐shading component, which provides an algorithm for inter‐plant competition based on crown zone areas. This model is termed the ‘Conductance model’ and this article outlines its development, applications to date, goodness of fit to experimental data, and discusses its strengths and weaknesses and scope for further testing and application. This article, which is dedicated to the late David Aikman, also sets out how the model can be applied to simulating weed–crop competition from simple data sets.     

Oxidative stress and antioxidative responses in plant–virus interactions

During plant–virus interactions, defence responses are linked to the accumulation of reactive oxygen species (ROS). Importantly, ROS play a dual role by (1) eliciting pathogen restriction and often localized death of host plant cells at infection sites and (2) as a diffusible signal that induces antioxidant and pathogenesis-related defence responses in adjacent plant cells. The outcome of these defences largely depends on the speed of host responses including early ROS accumulation at virus infection sites. Rapid host reactions may result in early virus elimination without any oxidative stress (i.e. a symptomless, extreme resistance). A slower host response allows a certain degree of virus replication and movement resulting in oxidative stress and programmed death of affected plant cells before conferring pathogen arrest (hypersensitive response, HR). On the other hand, delayed host attempts to elicit virus resistance result in an imbalance of antioxidative metabolism and massively stressed systemic plant tissues (e.g. systemic chlorotic or necrotic symptoms). The final consequence of these processes is a partial or almost complete loss of control over virus invasion (compatible infections).     

The spread oi Erysiphe graminis f. sp. hordei in mixtures of barley varieties

A model is proposed of mechanisms which might affect the progress of Erysiphe graminis f. sp. hordei in mixtures of barley varieties. Results obtained from two field trials indicate that the efffect of mixtures may be panitioned into three categoriesof the influence of the reduced density of the susceptible plants, the barrier effect of the resistant plants, and the induced resistance due to the non-virulent pathogen biotypes. In the early stages of plant growth the lower density of susceptible plants accounted for most of the reduction in pathogen development in mixtures. As the epidemic progressed, the barrier and induced resistance effects increased in importance and the total mixture effect was at a maximum mid-way through epidemic development. Towards the end of the trials the overall mixture effect declined though the influence of induced resistance was at its maximum. The reasons for these changes and their implications for the use of host varietal mixtures in disease control are discussed.
Mixtures also protected the crop against a pathogen other than the target organism.     

Comparison of an in vitro and a damping-off assay to test soils for suppressiveness to Pythium aphanidermatum

Testing of soil samples in greenhouse assays for suppressiveness to soilborne plant pathogens requires a considerable investment in time and effort as well as large numbers of soil samples. To make it possible to process large numbers of samples efficiently, we compared an in vitro growth assay with a damping-off assay using Pythium aphanidermatum as the test organism on tomato seedlings. The in vitro test compares the radial growth or relative growth of the fungus in soil to that in autoclaved soil and reflects suppressiveness of soils to the pathogen. We used soils from a field experiment that had been farmed either organically or conventionally and into which a cover crop (oats and vetch in mixture) had been incorporated 0, 10, 21, and 35 days previously. We obtained a significant, positive correlation between damping-off severities of tomato seedlings in damping-off assays and both relative and radial growth in vitro. In addition, radial and relative growth of P. aphanidermatum in the in vitro assay were positively correlated with several carbon and nitrogen variables measured for soil and incorporated debris. We did not find differences between the two farming systems for either growth measures of P. aphanidermatum or disease severities on tomato at different stages of cover crop decomposition. The in vitro assay shows potential for use with any fungus that exhibits rapid saprophytic growth, and is most suitable for routine application in suppressiveness testing.     

Ausbreitung des parasitischen Unkrauts Phelipanche ramosa in der deutschen Landwirtschaft

The root parasitic weeds Phelipanche ramosa (branched broomrape) and P. aegyptiaca have the widest host range among Orobanche and Phelipanche species. In Western Europe, P. ramosa attacks, with increasing aggressiveness, crops such as oilseed rape, tobacco, hemp, and tomato. The unique biology of root parasites, establishes a closed link with their host plant, thus reducing the possibility to successfully control them. Control measures include (a) physical processes (such as weeding, solarization, deep ploughing, burning off, flooding), (b) chemical (like soil fumigation, use of herbicides, germination stimulants) and (c) biological methods (e.g. resistant varieties, use of fungi and insects as antagonists, trap and catch crops). German tobacco growers rely mostly on the herbicide method. They apply glyphosate in very low concentrations, when the first tubercles are formed. Also a fungal antagonist against the parasitic weed on tobacco was found in Germany, but until now has not been developed into a commercial mycoherbicide. After hemp production lost its significance as a crop in Germany, tobacco remained as the main host for P. ramosa. In the past 10 years, branched broomrape has spread out and currently it can be found in areas where previously were free of it. Since the elimination of EU subsidies, some tobacco growers began to cultivate on their land parsley instead of tobacco. As a result, parsley has now been infected with P. ramosa. When used 10 years ago as catch crop, parsley had a rather small effect on branched broomrape. This potential danger, especially by other potential host plants, such as oilseed rape, tomato and potato or even weeds should not be underestimated. Spread and expansion of the host plant spectrum of branched broomrape in Germany might be reduced by the introduction of appropriate phytosanitary measures and improved information policies.     

Crop architecture and crop tolerance to fungal diseases and insect herbivory. Mechanisms to limit crop losses

Plant tolerance to biotic stresses (mostly limited here to fungal pathogens and insects) is the ability of a plant to maintain performance in the presence of expressed disease or insect herbivory. It differs from resistance (the capacity to eliminate or limit pests and pathogens by genetic and molecular mechanisms) and avoidance (the ability to escape infection by epidemics). The ways to tolerance of pests and diseases are multiple and expressed at different scales. The contribution of organs to the capture and use of resources depends on canopy and root architecture, so the respective locations of disease and plant organs will have a strong effect on the crop’s response. Similarly, tolerance is increased when the period of crop sensitivity lies outside the period within which the pest or pathogen is present. The ability of the plant to compensate for the reduced acquisition of resources by the production of new organs or by remobilization of reserves may also mitigate biotic stress effects. Numerous examples exist in the literature and are described in this article. Quantification of tolerance remains difficult because of: (i) the large number of potential mechanisms involved; (ii) different rates of development of plants, pests and pathogens; and (iii) various compensatory mechanisms. Modelling is, therefore, a valuable tool to quantify losses, but also to prioritize the processes involved.     

Use of Green Fluorescent Protein-Transgenic Strains to Study Pathogenic and Nonpathogenic Lifestyles in Colletotrichum acutatum

ABSTRACT Colletotrichum acutatum, which causes anthracnose disease on strawberry, can also persist on several other plant species without causing disease symptoms. The genetic and molecular bases that determine pathogenic and nonpathogenic lifestyles in C. acutatum are unclear. We developed a transformation system for C. acutatum by electroporation of germinating conidia, and transgenic isolates that express the green fluorescent protein (GFP) were produced. Details of the pathogenic and nonpathogenic lifestyles of C. acutatum were determined by using GFP-transgenic isolates. Major differences between colonization-mediating processes of strawberry and of other plants were observed. On the main host, strawberry, the germinating conidia formed branched, thick hyphae, and large numbers of appressoria were produced that were essential for plant penetration. In strawberry, the fungus developed rapidly, filling the mesophyll with dense mycelium that invaded the cells and caused necrosis of the tissue. In nonpathogenic interactions on pepper, eggplant, and tomato, the conidia germinated, producing thin, straight germ tubes. Appressoria were produced but failed to germinate and penetrate leaf tissue, resulting in epiphytic growth without invasion of the plant. Penetration of the plant occurred only several days after inoculation and was restricted to the intercellular spaces of the first cell layers of infected tissue without causing any visible damage. Much of the new fungal biomass continued to develop on the surface of inoculated organs in the nonpathogenic interaction. The differences in fungal development on strawberry compared with the other plant species suggest that signal molecules, which may be present only in strawberry, trigger appressorial germination and penetration of the primary host.     

Phenology and growth of Orobanche crenata Forsk (broomrape) in four legume crops

Studies were conducted in the field in 2 years comparing the phenology and growth of Orobanche crenata (Forsk) (crenate broomrape) in lentils (Lens esculenta L.) cv. Castellana, peas (Pisum sativum L.) cv. Orix, vetch (Vicia sativa L.) cv. comun and broadbean (Vicia faba L. cv. Alameda. First attachment of O. crenata to these crops took place 9–14 weeks after mid-November planting and earlier after later plantings, Differences in the first O. crenata attachment dates and in the duration of the underground growth period of O. crenata were much greater between years for any given crop than between crops in a given year: both attributes were apparently affected more by seasonal climatic conditions than by crop species, Furthermore, there was no consistent relationship found between crop growth stages and time after first attachment of the parante. This occurred in lentils and vetch while they were vegetative, in peas at late vegetative-early flowering stages, and at widely varying growth stages in broadbean, depending on planting dates and years. The maximum number of O. crenata plants successfully attached to each individual crop plant decreased in the order: peas > broadbean > lentil > vetch, with 21, 14, 10 and 8 per plant, respectively, averaged over the two seasons. Similarly, plant parasites: host dry weight ratio were 1.0, 0.7, 0.3 and 0.2 For each of these crops, respectively.     

Biology and integrated pest management for the banana weevil Cosmopolites sordidus (Germar) (Coleoptera: Curculionidae)

The banana weevil Cosmopolites sordidus (Germar) is the most important insect pest of bananas and plantains (Musa spp.). The larvae bore in the corm, reducing nutrient uptake and weakening the stability of the plant. Attack in newly planted banana stands can lead to crop failure. In established fields, weevil damage can result in reduced bunch weights, mat die-out and shortened stand life. Damage and yield losses tend to increase with time. This paper reviews the research on the taxonomy, distribution, biology, pest status, sampling methods, and integrated pest management (IPM) of banana weevil. Salient features of the weevil's biology include nocturnal activity, long life span, limited mobility, low fecundity, and slow population growth. The adults are free living and most often associated with banana mats and cut residues. They are attracted to their hosts by volatiles, especially following damage to the plant corm. Males produce an aggregation pheromone that is attractive to both sexes. Eggs are laid in the corm or lower pseudostem. The immature stages are all passed within the host plant, mostly in the corm. The weevil's biology creates sampling problems and makes its control difficult. Most commonly, weevils are monitored by trapping adults, mark and recapture methods and damage assessment to harvested or dead plants. Weevil pest status and control options reflect the type of banana being grown and the production system. Plantains and highland bananas are more susceptible to the weevil than dessert or brewing bananas. Banana production systems range from kitchen gardens and small, low-input stands to large-scale export plantations. IPM options for banana weevils include habitat management (cultural controls), biological control, host plant resistance, botanicals, and (in some cases) chemical control. Cultural controls have been widely recommended but data demonstrating their efficacy are limited. The most important are clean planting material in new stands, crop sanitation (especially destruction of residues), agronomic methods to improve plant vigour and tolerance to weevil attack and, possibly, trapping. Tissue culture plantlets, where available, assure the farmer with weevil-free material. Suckers may be cleaned by paring, hot water treatment and/or the applications of entomopathogens, neem, or pesticides. None of these methods assure elimination of weevils. Adult weevils may also invade from nearby plantations. As a result, the benefits of clean planting material may be limited to a few crop cycles. Field surveys suggest that reduced weevil populations may be associated with high levels of crop sanitation, yet definitive studies on residue management and weevil pest status are wanting. Trapping of adult weevils with pseudostem or corm traps can reduce weevil populations, but material and labour requirements may be beyond the resources of many farmers. The use of enhanced trapping with pheromones and kairomones is currently under study. A combination of clean planting material, sanitation, and trapping is likely to provide at least partial control of banana weevil.Classical biological control of banana weevil, using natural enemies from Asia, has so far been unsuccessful. Most known arthropod natural enemies are opportunistic, generalist predators with limited efficacy. Myrmicine ants have been reported to help control the weevil in Cuba, but their effects elsewhere are unknown. Microbial control, using entomopathogenic fungi and nematodes tend to be more promising. Effective strains of microbial agents are known but economic mass production and delivery systems need further development.