Aug 6, 2020
Questions and answers on insect vectors of X-disease phytoplasma

Interview with Tobin Northfield, Washington State University Entomology.

1. What are the key insect vectors of X-disease phytoplasma?

Six leafhoppers are known to transmit X-disease phytoplasma: Colladonus montanus, Fiebriella florii, Scaphytopius acutus, Paraphlepsius irroratus, Colladonus reductus, and Colladonus geminatus. The three  most common in Washington are C. reductus, C. geminatus and to a lesser extent C. montanus.

2. Could other insects be transmitting the phytoplasma? Such as bees, aphids, etc.

It is very unlikely. For a phytoplasma to be transmitted it has to make it through the gut wall, survive the insect’s immune system, and make its way to the salivary glands to be transmitted. This path is difficult and strongly limits the range of species capable of spreading each phytoplasma species. Similarly, [the phytoplasma] is not able to pass through pollen, or seeds.

3. How do we trap for leafhoppers?

Use yellow sticky cards. Place them in the orchard after harvest about 3-5 ft from the orchard floor. Place traps on orchard borders, in areas of concern in your block and throughout block. Approx. 1 trap per two acres. Check traps every 1-2 weeks. Smaller orchards should use more traps. Use presence (an average of 1 leafhopper per trap) as a threshold to spray.

4. Do we know for sure that other leafhoppers are not vectors?

In the >60 years of testing vectors, first by the Wenatchee entomologist, Homer Wolfe in the 1950s, and subsequently by researchers in Washington, California, and Utah, among others, only 6 species have been identified. The more common the leafhopper species, the more likely it has been tested without successful transmission. However, we can’t be certain that no other species are able to vector X disease phytoplasma.

5. What happens if we spray aggressively no matter the leafhoppers species! Should I consider IPM under such risk? 

If leafhoppers don’t have or can’t transmit the phytoplasma, they are not important. They are not a huge concern by themselves. If you are spraying and numbers don’t seem to go down, look to the nearby environment, or other parts of the orchard. This may be particularly important when parts of the orchard are being mowed, or the ground cover is disturbed and they move. Increased capture in traps may be due to increased movement (only moving insects bump into traps), rather than increased numbers, so keep in mind disturbances that could suddenly increase numbers in traps.

To start to use IPM reduce alternative hosts, like broadleaf weeds. Kaolin clay sprays and reflective mulch are being examined as deterrents which can be tried and may help us go back to IPM soon.

6. Are there any cover crop management practice to reduce leafhopper presence? 

While leafhoppers can feed on grasses, grasses are not phytoplasma hosts. So, maintaining a healthy grass cover may actually reduce the number of weedy hosts. Until we better understand the phytoplasma host range, avoid broadleaved cover crops if possible, and particularly perennials that might maintain the phytoplasma from one year to the next.

7. Are these vectors also moving to other neighbor crops? Potato, alfalfa, mint, hemp, asparagus?

Most leafhoppers seem to prefer to have a diverse diet, but we don’t know yet what all crops these leafhoppers will or prefer to feed on.

8. Do we need to do a full canopy spray for leafhoppers or is the lower part enough?

We don’t know yet. They seem to feed on leaves on the tips of branches, that are more tender. For this reason, among others, suckers emerging from removed trees are important to remove. In previous research in Oregon and California leafhoppers were caught more below five feet. The one exception is a study in California where the orchard had been abandoned and species that are less abundant here (C. montanus and F. florii) were caught high in the canopy. When collecting C. geminatus and C. reductus, we are much more likely to collect them from ground cover than trees, so they may be more likely to occur in the lower part of the canopy near these alternative hosts. We have seen the leafhoppers of interest as far as 6 ft up into the tree so until we learn more, it would be best to spray the whole tree.

 9. Which weeds are most important?

Focus on removing perennials, because while annuals can propagate phytoplasmas, they must be re-infected by a leafhopper every year. White clover, alfalfa, dandelion, and mallow are perennial plants that we are currently evaluating as leafhopper hosts. We are focusing on perennial weeds because those are the hosts that could keep the pathogen in the orchard from year to year. It is important to remember grasses are not a phytoplasma host.

10. What are the best organic options?

Options are currently limited. WSU is trialing some options now, such as Surround and Extenday, as well as some sprays.

11. Should I be more concerned with adults or nymphs?

Adults may be more important for transmitting the phytoplasma. It takes 30 days from acquisition of the phytoplasma through feeding to transmission to a new plant. Nymphs are not likely to be important vectors yet, because they don’t move far and may not have lived with the phytoplasma long enough to transmit it. If you can prevent nymphs from maturing, though, that could be an effective control strategy over time.

To read online click here

Additional Information

Cody Molnar, WSU ITT Extension


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