Historically, our Areawide Pheromone and Sticky Trap monitoring for insects was terminated around the first of April as the produce season ended. Beginning 2 years ago however, we continued our Areawide Trapping Network throughout the summer to collect trapping data from all 16 areawide trap locations year-round. So why is this additional trapping data useful? For several reasons. First, understanding the activity of some of our key pests when produce is not grown during the summer may give us an indication of what to expect as the fall produce season begins. This may be particularly helpful for predicting moth flights and whitefly flights in August-September coinciding with early transplanting and direct seeded crops. Another example is keeping track of corn earworm which can unexpectedly show up near the beginning of fall harvests. Secondly, trapping for pests during the summer has shown us that 2 of our more important produce pests are not caught in traps during the summer. We presume this is due to the absence of brassica crops and weeds for diamondback moth, and high daytime/nighttime temperatures lethal to aphids. The fact that trap catches resume in the fall supports our conclusion that these pests are absent in the summer, only to reenter the desert via winds and/or transplants in the fall. And finally, it gives me something to do in the summer. So, visit the Areawide Summer Trap Network if you’re curious what our key pests are up to. We also continue to monitor thrips activity during the summer. This involves plant sampling using a dislodgement sampling method (beat pan) to determine the relative abundance of thrips adults and larvae on alfalfa, cotton, melons, wheat, Sudan grass, and weeds. This is being conducted to supplement our yellow sticky trap data that only indicates thrips adult flight movement an area. With this plant sampling we should be able to determine the primary host plants thrips are colonizing in the cropping season during the long hot summer. We are also trying to determine whether these crops allow for the reproduction of thrips. So far, all the crops and weeds we have sampled have shown that thrips will reproduce and complete their life cycle on them. This is important, particularly for weeds, as we are trying to determine whether INSV can be survive the summer in the absence of lettuce. It is also important because it allows us to determine potential exposure of thrips to key insecticides like Radiant and Lannate for resistance management purposes. These crops essentially serve as untreated refugia that likely sustains insecticide susceptibility and is the reason these products are still effective against thrips. We will be conducting this work throughout the summer, as well as through next produce season. We will periodically provide updates of these thrips monitoring results throughout the summer. As my dad used to say, “to solve the problem, you must first understand the problem”.
Fungal transmission of viruses:
Several viruses have been shown to be transmitted by soil-inhabiting fungi and protists. Two of the common fungal species that transmit viruses in vegetable cropping system are Olpidium spp. and Polymyxa spp. There is specificity between the zoospores of the fungi and virus particles for successful transmission to occur. Not all soilborne fungi can transmit viruses.
Olipdium brassicae is an obligate parasite, meaning the fungus always needs a host plant to survive (brassicae, cucumber, carrot, lettuce). The fungius that act as vector of s viruses survive the time in between crops by producing resting spores. A virus that we often see in the desert, Lettuce big vein virus, is transmitted fungus Olpidium brassicae . Lettuce big vein virus is acquired by the zoospore (motile spores) as well as resting spores of the fungus.
The other soilborne virus we see in the desert it Tomato bushy stunt virus is soilborne but has no known vectors. Thought it was predicted to be transmitted by a fungus Olpidium brassicae, new research findings have rules out the particular fungal vector. But there is a possibility that the virus has some soilborne vector.
Two genera of viruses; Nepoviruses (Nematode transmitted polyhedral viruses) and Tobraviuses (Tobacco rattle virus) are transmitted by nematodes. Xiphinema (Dagger nematodes), Longidorus (needle nematodes), Trichodorus and Paratrichodorus (stubby root nematode) vector transmit different species of viruses.
Virus transmission by nematodes happens in 7 different but interrelated steps.
1. Ingestion: The process where the nematode ingests the virus particle along with the plant material/sap.
2. Acquisiton: The unique receptor in the nematode identifies the virus, so he virus particles stay intact within the nematode.
3. Adsorption: The phenomena of the virus being intact instead of disintegrated in the nematode digestive system.
4. Retention: Once adsorption, the virus particle can be retained in nematodes anywhere from months to even years.
5. Release: The virus particles are released onto new plants when nematodes commences feeding on new plant.
6. Transfer: The virus particles get transferred to new plant.
7. Establishment: The virus particles establish themselves in the new host plant.
It is a sophisticated and thankfully a very specific relationship. Not all nematodes transmit virus, neither do nematodes vector all plant viruses. Nematodes and nematode transmitted viruses are of a bigger problem in perennial agriculture system such as citrus orchards and grapevines.
Viruses transmitted by nematodes:
tobacco rattle virus: beans, beets, peppers, potatoes, and spinach and 100 others tomato ringspot virus-tomato, cucumbers, peach rosette mosaic virus: peach, blueberries cherry
rasp leaf virus: cherry, apple
grapevine fanleaf virus: grapes and grape hybrids
Controlling Disease and Weeds with Band-Steam – Yuma Trials Show Good Promise
In previous articles (Vol. 11 (13), Vol. 11 (20), Vol. 11(24)), I’ve discussed using band-steam to control plant diseases and weeds. Band-steaming is where steam is used to heat narrow strips of soil to temperature levels sufficient to kill soilborne pathogens and weed seed (>140 °F for > 20 minutes). The concept is showing good promise. This past season, three trials were conducted examining the efficacy of using steam for disease and weed control in Yuma, AZ. In the studies, steam was applied in a 4-inch-wide by 2-inch-deep band of soil centered on the seedline using a prototype band-steam applicator (Fig.1). The band-steam applicator is principally comprised of a 35 BHP steam generator mounted on top of an elongated bed shaper. The apparatus applies steam via shank injection and from cone shaped ports on top of the bed shaper.
Trial results were very encouraging as the prototype applicator was able to raise soil temperatures to target levels (140°F for >20 minutes) at viable travels speeds of 0.75 mph. Steam provided better than 80% weed control and significantly lowered hand weeding time by more than 2 hours per acre (Table 1). Results also showed that Fusarium colony forming units (CFU) were reduced from 2,600 in the control to 155 in the 0.75 mph and 53 in the 0.5 mph treatments, respectively (a more than 15-fold reduction). A significant difference in Fusarium wilt of lettuce disease incidence was not found, however disease infection at the field site was low (< 2%) and differences were not expected. At 0.5 mph, fuel costs were calculated to be $238/acre which was considered reasonable and consistent with the values reported by Fennimore et al. (2014).
An unexpected finding was that plants in steam treated plots appeared to be healthier and more vigorous than untreated plots (Fig. 2). This trial is still in progress and it will be interesting to see if this improved early growth translates into increases in crop yield.
In summary, early trial results are showing good promise for use of band-steam as a non-herbicidal method of pest control. We plan on conducting further trials in this multi-year study. If you are interested in evaluating the device on your farm and being part of the study please contact me. We are particularly interested in fields with a known history of Fusarium wilt of lettuce and/or Sclerotinia lettuce drop that will be planted to iceberg or romaine lettuce.
As always, if you are interested in seeing the machine operate or would like more information, please feel free to contact me.
Acknowledgements
This work is supported by Crop Protection and Pest Management grant no. 2017-70006-27273/project accession no. 1014065 from the USDA National Institute of Food and Agriculture, the Arizona Specialty Crop Block Grant Program and the Arizona Iceberg Lettuce Research Council. We greatly appreciate their support. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.
A special thank you is extended to Mellon Farms for allowing us to conduct this research on their farm.
References
Fennimore, S.A., Martin, F.N., Miller, T.C., Broome, J.C., Dorn, N. and Greene, I. 2014. Evaluation of a mobile steam applicator for soil disinfestation in California strawberry. HortScience 49(12):1542-1549.
Click link below or picture to see the band-steam and co-product applicator in action!
Carryover of Vegetable Herbicides to Wheat Grown in Rotation
Almost all the herbicides used on lettuce, cole crops and melons have restrictions on how soon wheat can be planted in rotation after they have been used. Experience has demonstrated, however, that safe intervals can vary considerably based upon many factors and are almost always much longer than they need to be. The most important factors are rate applied, irrigation practices and tillage. For example, when Kerb used to be banded at 2 to 4 lbs. per acre after planting and incorporated with furrow irrigation, it was common to see treated strips across wheat fields which followed. This is uncommon now that lower rates are Chemigated. We still see some Balan injury at ends of fields or in overlaps especially when sudan is planted. Wheat it not very sensitive to Prefar and carryover injury is uncommon.
