Host mixture efficacy in disease control: effects of lesion growth analyzed through computer-simulated epidemics

Lesion growth varies among foliar parasites and in order to study the effect of lesion growth on the efficacy of host mixtures to control epidemics, we altered the epidemic simulator Epimul by integrating a lesion growth function into the model. A theoretical study was performed by simulating epidemics caused by parasites with different lesion growth rates, spore dispersal gradients and multiplication rates. We found that increases in lesion growth rates resulted in large decreases in the effectiveness of mixtures for disease control and interacted strongly with parasite multiplication rate and spore dispersal gradient. The decline in mixture efficacy for epidemics with high lesion growth rates was reduced when parasite multiplication rate was higher and spore dispersal gradient steeper. Our results suggested that the lower number of infections on susceptible plants in the mixture as a result of inoculum loss on resistant hosts was partially compensated by lesion growth.     

Developing smarter host mixtures to control plant disease

Adaptation of plant pathogens to disease control measures (both chemical and genetic) is facilitated by the genetic uniformity underlying modern agroecosystems. One path to sustainable disease control lies in increasing genetic diversity at the field scale by using genetically diverse host mixtures. In this study, a robust population dynamics approach was used to model how host mixtures could improve disease control. It was found that when pathogens exhibit host specialization, the overall disease severity decreases with the number of components in the mixture; this finding makes it possible to determine an optimal number of components to use. In a simple case, where two host varieties are exposed to two host‐specialized pathogen species or strains, quantitative criteria for optimal mixing ratios are determined. Using these model outcomes, ways to optimize the use of host mixtures to decrease disease in agroecosystems are proposed.     

Induced resistance for plant disease control: maximizing the efficacy of resistance elicitors

ABSTRACT Plants can be induced to develop enhanced resistance to pathogen infection by treatment with a variety of abiotic and biotic inducers. Biotic inducers include infection by necrotizing pathogens and plant-growth-promoting rhizobacteria, and treatment with nonpathogens or cell wall fragments. Abiotic inducers include chemicals which act at various points in the signaling pathways involved in disease resistance, as well as water stress, heat shock, and pH stress. Resistance induced by these agents (resistance elicitors) is broad spectrum and long lasting, but rarely provides complete control of infection, with many resistance elicitors providing between 20 and 85% disease control. There also are many reports of resistance elicitors providing no significant disease control. In the field, expression of induced resistance is likely to be influenced by the environment, genotype, and crop nutrition. Unfortunately, little information is available on the influence of these factors on expression of induced resistance. In order to maximize the efficacy of resistance elicitors, a greater understanding of these interactions is required. It also will be important to determine how induced resistance can best fit into disease control strategies because they are not, and should not be, deployed simply as "safe fungicides". This, in turn, will require information on the interaction of resistance elicitors with crop management practices such as appropriate-dose fungicide use.     

Modelling the effect of partial resistance on epidemics of downy mildew of grapevine

European Journal of Plant Pathology - The cultivation of grape varieties with partial resistance to disease may become an important component for disease management in the future. The impact of...     

Effects of induced resistance on infection efficiency and sporulation of Puccinia striiformis on seedlings in varietal mixtures and on field epidemics in pure stands

An inducer race ofPuccinia striiformis inoculated two days before a challenger race on wheat seedlings of cv. Clement, reduced infection efficiency by 44% and lesion expansion by 7.7%; there was no effect on sporulation rate. In field epidemics, induced resistance restricted the disease intensity on cvs. Clement and Austerlitz in pure stands by 44% and 57% respectively. In the glasshouse at the seedling stage, disease intensity in a varietal mixture was reduced by 82% compared to the susceptible pure stands; one third of the total disease reduction was attributable to induced resistance.     

Patterns of disease and host resistance in spatially structured systems

We use data from species of the anther-smut fungi and the host plants Lychnis alpina and Silene dioica to show that spatial structuring at different scales can influence patterns of disease and host resistance. Patterns of disease and host resistance were surveyed in an archipelago subject to land-uplift where populations of S. dioica constitute an age-structured metapopulation, and in three contrasting areas within the mainland range of L. alpina, where population distributions range from continuous, through patchy but spatially connected to highly isolated demes. In S. dioica, disease levels depend on the age, size and density of local patches and populations. Disease is most predictably found in larger dense host patches and populations of intermediate age, and more frequently goes extinct in small old populations. The rate of local disease spread is affected by the level of host resistance; S. dioica populations showing an increase in disease over time are more susceptible than populations where the disease has remained at low levels. Among-population variation in resistance is driven by founding events and populations remain differentiated due to limited gene flow between islands. As observed in the L. alpina system, when populations are more connected, a greater fraction of populations have disease present. Results from a simulation model argue that, while increased dispersal in connected systems can increase disease spread, it may also favour selection of host resistance which ultimately reduces disease levels within populations. This could explain the observed lower disease prevalence in L. alpina in regions where populations are more continuous.     

Spatiotemporal effects of cultivar mixtures on wheat stripe rust epidemics

The use of cultivar mixtures is increasingly practical in wheat stripe rust management. Field experiments with wheat cultivar mixtures were conducted to determine their effects on temporal and spatial patterns of stripe rust epidemics in three regions. In the Beijing and Gangu fields, where the epidemics were caused by artificial inoculation, disease incidence and the area under the disease progress curve (AUDPC) of the cultivar mixtures were significantly lower (P < 0.05) than those of the susceptible pure stands. We defined the relative effectiveness of cultivar mixture on disease development related to that in pure stands (REM). The results demonstrated that in many treatments of mixtures of susceptible cultivar with resistant cultivars at various ratios in different locations, their effects on disease reduction were positive (REM < 1). The reduction of epidemic rate in cultivar mixtures expressed in either early season or late season depended on the initial pattern of disease and cultivar mixture treatments. Semivariograms were used to determine the spatiotemporal patterns of disease in the Gangu field. The spatial analysis showed clear spatial patterns of the disease in all four directions of the fields on susceptible pure stands but not on cultivar mixtures. The results implied that the mechanisms of cultivar mixture on disease management might include the interruption of disease spatial expansion and a physical barrier to pathogen inoculum by resistant cultivars.     

甜菜夜蛾核型多角体病毒对宿主种群的控制作用

室内试验表明,甜菜夜蛾核型多角体病毒对宿主种群有明显的控制作用.当饲毒浓度在1.14×108PIBs/mL以上时,能引起97%的甜菜夜蛾幼虫死亡;饲毒后1～8天内,致死中浓度LC50值在8.4718×105～2.6098×10 12PIBs/mL之间;在饲毒浓度范围内,致死中时间LT50在1.98～6.99天之间;感病幼虫每日病死时间分布可用时间-剂量-条件死亡率模型较好地拟合,累计病死时间分布可用Gompertz模型拟合.室内试验还表明,病毒对甜菜夜蛾种群有明显的亚致死作用.免于病死蛹重量减轻,羽化所得雌成虫产卵量下降,产卵期延长;从感病高龄幼虫发育的成虫能将病毒传播至子代,引起20%～48%的子代幼虫死亡,并使子代成虫产卵量下降.田间试验表明,病毒除具有明显的控制作用外,还有一定的后效作用.     

小麦黄矮病不同发生时期对产量影响及其防治对策

介绍了小麦黄矮病的发病因素、发病症状及不同时期、不同程度地发病对小麦产量的影响,阐明了最佳防治时间和防治方法.     

猕猴桃细菌性花腐病及其防治

我国猕猴桃大规模商品化栽培已有20多年历史, 同其他作物一样,随着从野生到大面积人工栽培的时间延长, 其病虫害种类和发病程度呈现复杂化趋势.湖北省是猕猴桃原生地分布省份之一, 现人工栽培面积达2 000 hm2.     

The use of host resistance in disease management of rust in common bean

Bean rust, caused by Uramyces appendiculatus, is one of the major diseases in dry and snap bean production world-wide. Numerous advancements in disease management have been made to reduce rust losses. Host resistance is an important component of rust management. However, durability of disease resistance has often been short due to the use of single genes for resistance interacting with extremely high virulence diversity of the bean rust fungus. The challenge to increase durability of resistance has led to strategies such as gene pyramiding of race-specific resistance, selection and use of partial resistance, and investigation and discovery of leaf morphological features that may slow the rust epidemic. Germplasm with multiple sources of rust resistance has been developed in specific bean seed classes and released for public and commercial use in intensive production systems such as those in the United States. However, progress to develop rust resistant germplasm for the subsistence agriculture of Latin America and Africa where intercropping and mixed cultivars dominate the production system has been slow. Incorporation of high yielding, disease-resistant components as partial replacement in farmer's mixtures has the potential to reduce severity in the crop and increase yield in the presence of rust. This strategy would not erode the genetic diversity that is historically known to enhance resistance durability and for many years has given stability in production in the subsistent agriculture systems.     

Modelling the interaction between powdery mildew epidemics and host dynamics of tomato

European Journal of Plant Pathology - A model simulating the progress of powdery mildew coupled to the growth dynamics of tomato, with a time step of 1&;nbsp;day, is developed. The model is...     

Long‐distance dispersal and its influence on adaptation to host resistance in a heterogeneous landscape

Small propagules like pollen or fungal spores may be dispersed by the wind over distances of hundreds or thousands of kilometres, even though the median dispersal may be only a few metres. Such long‐distance dispersal is a stochastic event which may be exceptionally important in shaping a population. It has been found repeatedly in field studies that subpopulations of wind‐dispersed fungal pathogens virulent on cultivars with newly introduced, effective resistance genes are dominated by one or very few genotypes. The role of propagule dispersal distributions with distinct behaviour at long distances in generating this characteristic population structure was studied by computer simulation of dispersal of clonal organisms in a heterogeneous environment with fields of unselective and selective hosts. Power‐law distributions generated founder events in which new, virulent genotypes rapidly colonized fields of resistant crop varieties and subsequently dominated the pathogen population on both selective and unselective varieties, in agreement with data on rust and powdery mildew fungi. An exponential dispersal function, with extremely rare dispersal over long distances, resulted in slower colonization of resistant varieties by virulent pathogens or even no colonization if the distance between susceptible source and resistant target fields was sufficiently large. The founder events resulting from long‐distance dispersal were highly stochastic and exact quantitative prediction of genotype frequencies will therefore always be difficult.     

油菜内生生防菌BY-2在油菜体内的定殖与对油菜菌核病的防治作用

 从油菜植株体内分离出的内生细菌BY-2,经过生物学鉴定为枯草芽孢杆菌(Bacillus subtilis)。用BY-2回接油菜,重新分离到具有BY-2相同形态特征和抑制病原真菌能力的内生菌株。油菜接种后的第10 d,体内的BY-2菌数达(2.24~9.02)×10³ cfu/g鲜植株,25 d仍然保持在(3.13~8.59)×10³ cfu/g鲜植株。BY-2与油菜核盘菌[Sclerotinia sclerotiorum(Lib.) de Bary]对峙培养可以形成直径为3.1 cm的抑菌圈;可使油菜核盘菌菌丝细胞浓缩变短,细胞壁破裂,原生质外溢,从而抑制真菌生长发育;同时还能抑制菌核的萌发,抑制率达60%~70%;在油菜离体叶片试验中,BY-2对菌核病的防治效果达100%。     

Asymptotic behaviour and threshold criteria in model plant disease epidemics

Two qualitative results concerning threshold criteria and asymptotic behaviour in plant disease epidemics are derived from Vanderplank's differential-difference equation. Analysis shows the dependence of these results on initial disease and clarifies some confusion in the literature. Results from the deterministic theory of medical epidemics, based on linked differential equations describing the dynamics of different categories of diseased individuals, are compared with the results derived from the differential-difference equation. Generally, the results correspond although the effects of initial disease need clarification. The need for a strict operational definition of the progeny-parent ratio limits its present use in plant disease epidemiology. In particular the numerical value of the ratio is not a sufficient basis for distinguishing between endemic and epidemic disease. There is a need to link theory in plant disease epidemiology with similar theory in other areas of population biology. The use of linked differential equations, rather than the differential-difference equation, provides a more flexible analytical tool.     

一种新的棉花黄萎病快速接种技术及其在病原菌致病力和寄主抗病性鉴定上的应用

 棉花品种抗黄萎病鉴定一般在田间病圃中进行,其结果受病圃中病原菌分布均匀程度、气象等因素影响极大,往往导致鉴定结果不准确。为了使鉴定方法简单、科学、可靠,我们在温室条件下比较了3种苗期接种棉花黄萎病的方法,即切根蘸孢子法(接种浓度为10⁶分生孢子/mL);菌培养物土壤接种法(0.5%、1%、2%,w/w);微菌核土壤接种法(10³个微菌核/g土)。结果表明,切根蘸孢子法导致棉苗发病均匀、严重、迅速,播种35~45 d后即可得到均匀一致的发病结果。而其它2种接种方法在播种75 d后才得到相对稳定的发病结果。同时,研究还表明接种浓度为10⁴分生孢子/mL所导致的黄萎病显著比10⁵或10⁶的轻。利用切根蘸孢子法在室内鉴定12个棉花品种或品系的抗黄萎病能力,证明该方法是抗黄萎病快速鉴定的有效方法。此外,该鉴定方法还可快速鉴定黄萎病菌不同菌株的致病性,并可应用于作物对其它土传病害的抗病性鉴定上。