Managing aboveground pests: Arthropod vectors of citrus pathogens: Part I February 13, 2019 Lecture Overview Part I: • Vector-borne disease concepts • Introduction to Disease Vectors • Taxonomy • Biology • Vector-borne diseases of citrus • Viral Pathogens Part II: • Vector-borne diseases of citrus • Bacterial Pathogens • Case studies: • Citrus Variegated Chlorosis and Citrus greening disease 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 damage/loss due to plant diseases Pathogen due to direct or indirect effects of (Vector) 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 Host/Pathogen Relationships • Environmental and physiological factors contribute to the development of disease • Resistance: ability of a host to prevent infection and disease • Virulence: ability of a pathogen to produce disease • Many plant pathogens need to be transmitted by a vector • A pathogen’s host range may be determined by the host range of the vector - it can only infect plants that the insect vector feeds upon Vector/Host Relationships • Generally, the closer the association between vector and host, the greater the suitability of the vector to transmit disease • Different degrees of association are possible 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) true VECTORS Vectors of plant disease • Mites • Insects • Nematodes Phylum : Nematoda • Roundworms • May transmit viral, bacterial and fungal plant diseases • Often mechanical: feeding causes injuries to roots which become the avenues for entrance of fungal and bacterial pathogens • Grapevine fanleaf (GFLV) virus of is a well known example of transmission by Xiphinema nematodes Class: Arachnids (Arachnida) • Four pairs of jointed legs • Two body segments: • Cephalothorax (fused head and thorax) • Non-segmented abdomen • No antennae Order: Acari (mites and ticks) Morphological characteristics: • Very small, sometimes microscopic, arthropods with an oval or elongated body. • The mouthparts, called chelicera, are adapted for piercing, sucking and lacerating. Class : Insecta (insects) • Three pairs of jointed legs • Three distinct body segments: head, thorax and abdomen • One pair of antennae Order: Hemiptera (aphids, hoppers, psyllids, etc.) Morphological characteristics: • Two pairs of wings, which form a roof- like structure at rest • The forewings and hindwings have a similar shape (hence the name "Homo-ptera"). • Short, hair-like antennae • Mouthparts designed for sucking or piercing Vectors species found primarily in the suborders Auchenorrhyncha and Sternorrhyncha Piercing/sucking mouthparts Pathogen location Aphid with stylet embedded in sieve tube 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 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 • Latent period: time between the acquisition and inoculation of a pathogen by the vector • Inoculation access period (IAP): time required for a vector to transfer a pathogen to a susceptible plant Main mechanisms of pathogen transmission by insects Non-persistent, stylet-borne: • 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 • Pathogens are often mechanically transmissible Main mechanisms of pathogen transmission by insects • Systemic/Biological: ingestion of pathogen with plant sap while eating. The pathogen circulates (passes through the gut into the hemolyph), may or may not multiply, and reaches the salivary glands and mouthparts through which it is subsequently injected into plants during feeding. • The pathogen either reproduces, undergoes developmental changes, or both in the vector • The most effective and significant mechanism for disease transmission by arthropods • Semi-persistent • Persistent 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 Persistent transmission • Insects can inoculate the acquired virus for long periods, transmitting the pathogen after molting and often for their entire lifespan (larvae or/nymphs into adults) • Can be circulative or propagative Persistent transmission • Circulative: • 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 Persistent 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) Persistent propagative transmission Hogenhout et al. 2008 Modes of plant pathogen transmission by vectors Barriers to persistent transmission 1. midgut infection 2. dissemination (including midgut escape and salivary gland infection) 3. salivary gland escape 4. transovarial transmission (vertical transmission) Passage of persistent viruses through different organs in their insect vectors requires specific interactions between a pathogen and vector components Types of vector-borne plant pathogens • Viruses • Bacteria • Mollicutes: Spiroplasmas and phytoplasmas • Proteobacteria Vector-borne citrus virus and virus-like diseases • Citrus tristeza virus (CTV): aphids ; the most famous and economically important citrus virus • Citrus Leprosis: brevipalpus mites • Vein enation: aphids It is widely distributed, but causes little economic damage. • Citrus chlorotic dwarf: bayberry whitefly (Parabemesia myricae) • Satsuma dwarf: soil-borne vector? • Citrus ringspot virus: aphids? • Citrus yellow mosaic: mealybugs • Etc. Citrus Leprosis • Problem on sweet orange varieties • Caused by virus that is not systemic in plant • Primarily a disease of oranges, but may affect mandarins • Distribution: highly important disease of citrus in South, Central, and North America (recently Panama) Citrus Leprosis: Causal agent Causal agent • Associated with two distinct rhabdovirus-like viruses, one residing in the cytoplasm and the other in the nucleus of infected cells • Virus particles can be observed by electron microscopy of lesions from naturally infected citrus and from mechanically inoculated Chenopodium quinoa (universal virus host) plants Citrus Leprosis: Symptoms • Chlorotic lesions on citrus leaves, fruit, and twigs • Chlorotic lesions eventually may become necrotic in the center • Flat or depressed lesions on fruit with concentric patterns and gumming • Abscission of leaves and fruit and twig dieback due to extensive lesion development • Yield reduction, decline and eventual death of citrus trees Leprosis: Leaf lesions Leprosis: Stem lesions Leprosis: Fruit lesions Leprosis: Fruit drop Citrus Leprosis: Vector • Vector: Brevipalpus mites (Family: Tenuipalpidae- flat or false spider mites) • Several species are reported to be vectors: B. californicus, B. obovatus and B. phoenicis • Only B. phoenicis has been confirmed to be an effective vector in experimental transmission studies • According to Knorr (1959) B. californicus was vector in Fl. No voucher specimens available. Citrus Leprosis: Transmission • Highest transmission efficiency occurs in larval stage rather than at nymphal or adult stages • No transovarial transmission • each newly hatched larvae needs to feed on infected plant tissues in order to acquire the virus • Evidence for persistent propagative transmission: • Transstadial transmission • Retain ability to transmit the pathogen after being held on unsusceptible host plants • Infective throughout lifespan Citrus Leprosis: the Florida situation • Prior to 1925, leprosis was a serious disease of citrus in Florida • Reports from 1906-1968: