It isn’t enough that plant pathogenic fungi, bacteria, viruses, viroids, phytoplasmas and nematodes can attack vegetable crops and cause diseases, and that parasitic plants such as dodder can do the same. Growers also have to worry about insects and pests that cause direct crop damage and increase plant susceptibility to disease-causing pathogens indirectly through the wounds they cause, and the weakening of the plant. And then there are those pesky direct-vectored diseases.
Pathogens which are direct-vectored by insects or other organisms are spread when the infected hosts feed on plant material, injecting the pathogen and beginning the disease cycle. Aphids, mites and nematodes often serve as plant pathogenic vectors. Not all species of these potential vectors carry plant pathogens, and plenty do carry plant pathogens in their bodies but are not capable of transmitting them directly to the plant. But those that do serve as direct vectors for plant pathogens can cause a lot of trouble.
Direct-vectored plant diseases can often spread quickly and widely. Some vectors can rapidly move from plant to plant and location to location, transmitting pathogens before any obvious damage, or even significant numbers of the vector organism, are seen. Depending on the type of transmission, other vectors can remain capable of disease transmission for long periods of time, rather than transmitting just once. In some cases, the presence of vectors allow the pathogen to multiply more rapidly than it otherwise would.
These direct transmitters of plant pathogenic disease organisms don’t all cause infections in the same manner. There are various requirements which different pathogens have of their host vectors. Some are just along for the ride; others need to circulate around the host first; others need to replicate inside the host before they can be transmitted.
Persistent or circulative transmission: The pathogen accumulates inside the vector, but does not replicate. The pathogen circulates through the vector and enters into the salivary glands. It is expelled into a new plant host when the vector feeds on the plant. The pathogen can be transmitted from the vector to the plant for days or weeks. The pathogen does have an initial dormant period in the vector prior to being transmittable. Vectors have to feed for a given time period before the pathogen can travel from the plant and infect them.
Non-persistent transmission: This occurs when aphids or other sucking organisms carry the pathogen in their mouthparts. The pathogen does not enter the host’s circulation, staying in the mouth or foregut only. The pathogen is then released back out of the vector and into the next plant on which it feeds. Transmission occurs only during a very short period of time after the vector initially acquires the pathogen, but the pathogen is also quickly picked up by vectors during feeding and transmitted rapidly from the vector to a new plant with no dormant period needed.
Propagative transmission: This is a type of persistent transmission. The pathogen enters the vector’s salivary glands and also propagates inside of the vector, increasing its population. The pathogens are then expelled into the host plant during vector feeding, and also propagates within the infected plant tissue.
Direct Vectors and Diseases
Aphids spread pathogens as they suck sap from plant tissue. There are many species and even if they aren’t vectoring a disease, honeydew – a result of their feeding – attracts fungi and bacteria, and the damage aphids and similar sap-sucking pests cause to the plant while eating allows these pathogens entry into the plant, indirectly vectoring diseases.
Some aphids directly vector plant pathogens too. Most direct-vectored aphid diseases are transmitted in a non-persistent manner, residing in the mouthparts of the aphid for a brief period. This also, however, allows aphids to acquire pathogens from an infected plant rapidly – and rapidly transmit it to the next plant as the aphid feeds.
Some of the diseases in which aphids are direct transmitters in a non-persistent manner are watermelon mosaic virus, zucchini yellow mosaic virus, turnip mosaic virus and lettuce mosaic virus. Persistent transmission of disease also occurs via aphids. Potato leaf roll virus, lettuce necrotic yellows virus and pea enation mosaic virus-1 are transmitted this way.
Cucumber beetles (striped and spotted) directly vector bacterial plant pathogens. Both serve as vectors for bacterial wilt of cucumber. The beetles carry bacteria in their mouthparts and deposit it into the plants as they feed.
Leafhoppers are also important insect vectors, directly transmitting several damaging diseases. Beet curly top virus is transmitted by the beet leafhopper, which carries the virus for life once it is acquired by feeding on an infected plant. The phytoplasma which causes aster yellows in a wide variety of crops is vectored by the aster leafhopper. This disease spreads via propagative transmission in the leafhopper. It requires a minimum of 11 days of incubation, once the leafhopper is infected, before it can be transmitted to another plant.
Flea beetles can carry pathogens in their mouthparts, directly vectoring bacterial wilt in cabbage and in grains, including Stewart’s wilt in sweet corn. They can also vector several diseases of potatoes.
Whiteflies, primarily those of the genus Bemisia, can vector plant pathogens and spread disease such as the tomato yellow leaf curl virus. The greenhouse whitefly is the only vector for tomato infectious chlorosis virus. This virus remains in the whitefly for several days, during which it is transmitted to plants during feeding. Other serious directly-vectored whitefly diseases include beet pseudo yellows virus.
Psyllid are also direct vectors of disease. The tomato and potato psyllid transmit the bacteria that cause the zebra chip disease of potato.
The two-spotted spider mite has been associated with and believed to vector bacterial fruit blotch, and is a prevalent pest of cucurbits. The disease is a significant economic concern.
Vectors which cause vegetable diseases can’t all be managed in a similar manner. Pesticide applications are tricky due to the various modes of transmission, which require different approaches in order to impact the targeted pest and decrease disease exposure.
Other control methods can have a significant effect on reducing pest populations and resulting disease concerns. Row covers, which exclude insects, can eliminate the spread of disease by preventing insect vectors from reaching the crop. Weed control can eliminate virus inoculant as many weeds can harbor these direct-vectored diseases, or provide host plants for the vectors themselves. Various types of mulches have been used to deter insects successfully, preventing vector populations from reaching threshold levels.
Biological controls, which focus on utilizing non-GMO insects, microbes or plants to control pest populations and prevent disease issues in crops, can decrease vector populations. Timing of biologicals is important, as they require certain conditions in order to thrive and prey upon pests during their susceptible lifecycle stage. Proper pest identification is crucial when selecting biological control methods.
Planting resistant varieties, rotating crops, companion planting, modifying planting times to avoid pest populations and selecting planting sites which don’t provide the proper environment for the targeted vector or pathogen all are means of reducing disease pressures. Using a variety of methods to reduce pest pressures is the best way to manage plant pathogenic diseases which are directly vector by pests.