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Insect Vectors of Plant Pathogens
Introduction
Thousands of plant pathogens are known to affect plants. Generally, these plant pathogens are viruses, but they can also be bacteria, fungi, or parasitic nematodes. Most of the vectors of plant pathogens are sucking insects feeding on plant juices of the suborder Homoptera under order Hemiptera. The small stylets of these insect vectors help in the insertion of pathogens to plant cells or vascular tissues with minimum physical damage.
Aphids (family Aphididae) are the most widely known vectors of plant pathogens. Whiteflies (Aleyrodidae), leafhoppers (Cicadellidae), plant hoppers (Fulgoridae), mealybugs (Pseudococcidae) and various other hemipteran insects help in the transmission of plant pathogens.
Transmission Plant Pathogens
Some plant viruses are also transmitted by thrips (Thysanoptera).
Other important insect vectors of plant pathogens include leaf beetle (Chrysomelidae), and weevil (Curculionidae).
Pathogens transmitted by beetles (order Coleoptera) are known to infect plants severely because of their feeding behaviour and damaging plant tissues. Insect feeding on the plant can damage tissues and wounds can be invaded by fungi.
Insects infested with the fungus while feeding can transfer them to the plant fruits. Nematospora corylii is an example of a fungus causing yeast spot disease in coffee, bean, cotton, and a variety of crops. Sucking insects feeding on grapevines can cause lesions on the roots, leading to invasion by soil fungi, and damaging the roots. Ophiostoma ulmi is an example of a fungus transmitted by beetles causing Dutch-elm disease.
Wood-boring beetles (Cerambycidae) are vectors of nematodes causing pine wilt disease. Nematodes divide and multiply inside plant tissues, crawl and enter inside the spiracles of beetles, which act as transporters of this pathogen to spread the infection to a healthy plant.
Common Insect Vectors of Plant Diseases
Type of Pathogen |
Pathogen |
Vector |
Disease and its description |
|---|---|---|---|
Nematode |
Pine wood nematode |
Long-horned beetles |
Browning/yellowin g of pine leaf needles |
Trypanosome s |
Hartrot of palm |
Shield bugs |
Leaf discoloration reaching to the canopy |
Fungus |
Yeast spot of bean or coffee Dutch elm fungus |
Hemipteran fruit/seed feeders Bark beetles |
Circular leaf spots with grey or white centres Leaves curl and wilt |
Bacteria |
Aster yellow Xylella fastidiosa |
Leafhoppers Xylem sap- feeders |
Stunted growth of plant, yellow leaves, distorted leaves,stems, and flowers Leaf scorching or browning |
Virus |
Southern bean mosaic virus Maize streak virus Maize rough dwarf virus Lettuce yellows virus Barley yellow dwarf virus Tomato yellow leaf curl virus Tomato spotted wilt virus Rice dwarf virus Lettuce mosaic virus Tobacco mosaic virus Cucumber mosaic virus Cauliflower mosaic virus Maize mosaic virus |
Leaf beetles Leafhoppers Planthoppers White fly Aphids White fly Thrips Leafhopper Aphids Aphids Aphids Aphids Planthopper |
Yellow or green mosaic appearance on leaves Chlorotic streaks on leaves, plant with stunted growth Stunted plant growth Browning of leaf veins, plant stunted and turns yellow Bright yellow coloration of leaves Small leaves and turns yellow Bronzing on leaves, chlorotic spots on fruit Leaves turns yellow at the tip and margins Leaves are stunted and deformed with mosaic appearance Yellow, white, or green stripes with mosaic appearance Yellow, white, or green stripes with mosaic appearance Yellow, white, or green stripes with mosaic appearance Light green or yellow streaks on leaves along the veins |
Control measures
The epidemiology of plant diseases caused by insect vectors is complex, it involves the plant, the pathogen, its vector, the environment, and their interaction in diverse ways. Therefore, an integrated approach is needed to get an appropriate outcome.
Chemical control can be used and it involves the application of pesticides to plants. It has good application in the control of potato leafroll and barley yellow dwarf spread by aphids.
A variation of chemical control can be the use of stylet oil, horticulture oils or insect repellents. But in an infected field with a high number of vectors pesticide treatment or other methods of chemical control may be ineffective.
The application of biological methods in the control of these vectors can be variable. In a small ecosystem with few varieties of crops and insect vectors, the natural enemies can be used effectively. But in areas with a high population of insect vectors and natural enemies, or cases of very low vector count it turns risky and causes economic losses.
Sometimes spraying of insecticides and pesticides also disrupt the natural enemies of the insect vectors. There can also be host plant resistance to insect vectors, which depends on the plant's efficacy and means of resistance. When a resistance repels an insect from feeding on the plant, it can prevent pathogen spread, but if the resistance prevents slow population growth, it cannot prevent pathogen infection. But complications arise in cases of host plant resistance, where the pathogen strains mutate and change into more infectious varieties.
Cultural methods like plant date adjustment, pruning, and removal of volunteer crops or non-crop reservoir hosts of vectors and pathogens can be effective measures for the management of diseases and can minimise plant exposure to these pathogens.
Adoption of regulatory measures like proper sanitation, quarantines, crop-free periods, insect vector monitoring and pathogen detection can be very effective to control plant pathogens and their vectors.
Some countries have regulations for plant seed testing for pathogen incidence before commercial use for agriculture.
Integrated pest management is the most viable and effective way for plant disease control and crop production. The production of potatoes is one such example.
In some varieties of potatoes, internal discolouration occurs due to infection of Potato leafroll virus (PLRV) and these potatoes are rejected. To prevent PLRV infection, clean propagation methods of potato tubers are followed and commercial growers survey for aphid growth. If the aphid number exceeds the economic threshold, the crops are treated with insecticide to prevent secondary infection.
Cultural methods are used to remove volunteer potatoes, peaches, and apricots trees vectored by the same insects. Inspection departments undergo regular checking and prevent the sale of infected potatoes.
Lastly, concerned government bodies make resistant potato varieties to LRV infection, but with good taste and high quality.
FAQs
Qns 1. Do insects transmit plant diseases?
Ans. Yes, insects act as a vector of pathogens to plant diseases.
Qns 2. Which insects transmit plant diseases?
Ans. Aphids, whiteflies, leafhoppers, planthoppers, thrips.
Qns 3. Are insect-vectored plant diseases curable?
Ans. Yes, they can be controlled and plant diseases can be eliminated.
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