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Managing Aboveground Pests: Arthropod Vectors of Citrus Pathogens: Part I February 17, 2021 Lecture Overview

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Managing aboveground pests: Arthropod vectors of citrus pathogens: Part I February 17, 2021 Lecture Overview Part I (this week): • Vector-borne disease concepts • Introduction to Disease Vectors • Taxonomy • Biology • Vector-borne diseases of citrus • Viral Pathogens Part II (next week): • Vector-borne diseases of citrus • Bacterial Pathogens • Case studies: • Citrus Variegated Chlorosis and Citrus greening disease Learning objectives: • Learn the mechanisms of insect transmission of plant pathogens. • Learn the terminology of insect transmission. • Think about the interaction of the plant, pathogen, and vector. Think about factors affecting transmission by insects. • Know the common groups of arthropods that transmit pathogens The most destructive plant diseases are vector-borne • Plant pathologists have a different point of view about vector- borne diseases than entomologists. • Plant pathology view: the most destructive plant pathogens are those which are transmitted by vectors • Disrupt the pathogen multiplication in plant host. • Entomology view: One insect can cause economic damage by transmitting a pathogen to plant. • Controlling the insect vector is the key to stopping the disease. • Disruption of pathogen acquisition by the insect from the plant may lead to total control of the disease. Economic Injury Level (EIL) - the lowest pest population level that will cause economic damage or the critical population density where the loss caused by the pest equals in monetary value to the cost of management. Economic Threshold or Action Threshold (ET) - the point at which management actions should be taken to prevent an increasing pest population from exceeding the economic injury level. The ET always represents a pest density or level of pest damage lower than the EIL. Plant disease epidemics • In special cases, where pests serve as vectors of plant or animal diseases, the economic threshold (ET) is zero. A single pest attack may cause the death of a plant or animal. • Among many factors affecting the epidemic of plant disease, vectors are the most important factor. • The vector is responsible for disease spread by visiting plants and feeding on specific tissue where the pathogen exists and multiplies. Plant disease epidemics • Can result in large loss of crop yields or decimate entire plant species (e.g. Dutch elm disease) • Approximately 30-40% of Pathogen damage/loss due to plant diseases (Vector) due to direct or indirect effects of transmission and facilitation of pathogens by insects • Three elements needed for disease to occur (‘disease Susceptible Conducive triangle’) host environment • When a pathogen requires a vector to be spread, the vector must be plentiful and active for an epidemic to occur Disease Triangle • Not shown: time (latent period) for disease development Plant disease epidemics Mechanisms of insect transmission of plant pathogens • The insect transmission of plant pathogens is a complex process that includes specific interactions among the plant host, the insect vector, and the pathogen. • The insect needs to acquire the pathogen from an infected plant and inoculate the pathogen into a healthy plant in order to complete the transmission process. Vectors • Vector: an organism that does not necessary cause disease by itself, but spreads the infection by conveying pathogens from one host to another. Vectors of plant disease • Mites • Insects • Nematodes Types of vectors: • Mechanical vectors: • pathogen does not multiply within mechanical vectors • physically transport the pathogen from host to host • Biological vectors: • pathogen must multiply within a biological vector before the biological vector can transmit the pathogen. Vector/Pathogen Relationships • The ability of a pathogen to survive and remain infective in or on a vector species is a critical factor in disease transmission • Passive • Vectors feeding in or walking through an infected plant area carry pathogen on their body • Generally an inefficient mechanism for disease transmission • Example: Citrus canker • Active (mechanical or biological) àtrue VECTORS Transmission types • Transmission types are classified depending on pathogen behavior within the vector. • Including: Circulation, Propagation, Persistence • We will cover all of these types in greater detail. Transmission: A complex process Four major components. 1. Pathogen behavior within the Pathogen vector: 1- Persistence 2- (Vector) Circulation 3- Propagation 2. Vector feeding behavior: 1- Specific feeding site 2- specific feeding tissue 3- Susceptible Conducive background matching host 3. Tritrophic specific environment interaction: 1- Plant-vector interactions 2- Pathogen vector interactions 3- Vector-plant interactions Disease Triangle 4. Factors affecting the transmission efficiency: Environmental Circulation Circulative pathogens Propagation Main mechanisms of pathogen transmission by insects Understanding how vectors transmit pathogens increases knowledge of their epidemiologies and needed for development of disease management • Non-persistent • Semi-persistent • Persistent Transmission types • Non-persistent transmission- These pathogens (viruses) are acquired from infected host plants through the probing behavior of the insect in very short periods of time. • The insect then transmits the virus by probing on a healthy plant. • Mouthpart-borne (foregut or stylets) and non- circulative. Main mechanisms of pathogen transmission by insects Non-persistent: • Retained by the vector mainly in the stylet (food canal) • Does not circulate in body • Vector feeds on infected plant and carries pathogen on mouthparts to a new plant during subsequent feeding • No latent period • Not retained through molt Transmission types • Semi-persistent transmission- Viruses are acquired and transmitted by feeding rather than by probing. • The longer the insect feeds, the greater the rate of transmission. • The virus is retained for a few days and varies with the virus/vector combination. Modes of plant pathogen transmission by vectors Semi-persistent (foregut-borne) transmission: • Transmitted by the vector from a few hours to a few days post acquisition • Short AAP, a function of the time it takes to reach the plant vascular tissue • Short retention time (24-48 h) • Lost at molt • Retained mainly in the foregut • No latent period • Many whitefly-transmitted viruses Foregut-borne pathogens adhere to the food canal of the stylets or higher up in the foregut Managing aboveground pests Transmission types • Circulative non-propagative transmission: Viruses are acquired and transmitted by the feeding rather than the probing behavior of the insect vector. • 8- to 12-hour latent period for the virus to get from the gut to the salivary glands • longer feeding periods increase the chance of acquisition. Circulative non-propagative transmission • No replication in vector (non-propagative) • Latent period may be a few hours to a few days • Retained in vector for days or weeks (hemocoel, organs) • For many persistently transmitted plant pathogens, particularly in those infecting non-vegetative crops (e.g. citrus), insect transmission is obligatory for the pathogen Transmission types • Circulative propagative pathogens include viruses and bacteria Circulative propagative transmission • Propagative: • Replication occurs in the vector • Long AAP • Latent period may be days or weeks before the vector is able to transmit the pathogen • Pathogen often retained for the lifespan of the vector (hemocoel, organs) • Often transmission to progeny occurs through infection of embryo or germ cells in the female (transovarial transmission) • Spiroplasma citri (Citrus stubborn disease) Non-circulative propagative transmission • The only example for this type of transmission is Xylella fastidiosa. • Foregut limited- attaches and forms a bacterial biofilm. That is different! • The bacterium is located in the vector foregut (ciberium and pre-ciberium) • Nymphs lose the bacterium after molting while adults maintain the bacterium for their whole life. • Nymphs and adults transmit X. fastidiosa • No latent period No transmission after molting • No transovarial transmission Transmission parameters • Transmission parameters • Acquisition Access Period (AAP): the period of time given for the vector to acquire the pathogen from plant. • Inoculation Access Period (IAP): the period of time given for the vector to transmit (inoculate) the pathogen to plant. • Latent Period: the time between acquiring the pathogen by the vector and the ability to transmit it to plant. Pathogen Transmission • Transmission: transfer of a pathogen from an infectious source to a susceptible host, includes acquisition and inoculation • Acquisition access period (AAP): time required for a vector to ingest a pathogen from an infected plant host • Image: https://www.sciencedirect.co m/science/article/abs/pii/S2 214574516300657 Pathogen Transmission • Latent period: time between the acquisition and inoculation of a pathogen by the vector • Inoculation time period or inoculation access period (IAP): time required for a vector to transfer a pathogen to a susceptible plant Transmission parameters • Transovarial Transmission: transmission of pathogen from female parent to offspring through the ovaries. • Sexual Transmission: transmission of pathogen between infected and uninfected vector adults during courtship. Viruliferous: vector that is capable of transmitting the virus to numerous plants within its lifetime once it acquired the virus. Insect Body Insect body:
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