Aphid Control: We mentioned in our previous update that foxglove aphid was beginning to colonize lettuce in the Yuma Valley. In the past two weeks aphid activity, and particularly foxglove aphid, has increased dramatically in our untreated control lettuce plots here at the Yuma Ag Center. Keep in mind, foxglove aphids can be difficult to locate in fields due to their patchy distribution. It is not unusual to sample dozens of plants and find nothing and then find 2-3 plants heavily colonized with foxglove aphids. Green peach aphid is generally more uniformly distributed among plants. They are also still prevalent in our lettuce, albeit not at the same densities as foxglove aphids. We’re also finding green peach aphid and cabbage aphid beginning to rapidly colonize young cabbage plants. Because all these aphid species are inherently different in their susceptibility to insecticides, the proper choice of product is important for achieving adequate control. For more information on insecticide alternatives for the various aphid species please view our Aphid Control Chart-2014.
Cabbage looper: Our pheromone trap captures of cabbage looper moths the past 2 weeks have been unusually high for this time of the year (see Areawide Trapping Network). This is largely due to the above average temperatures we’ve been experiencing in the desert. In addition, corn earworm trap catches remain consistent from our last update, particularly in the Wellton/Dome Valley area. These increases in moth activity may indicate potential increases in egg laying and larval infestation in your later produce fields. It would be well advised to pay careful attention for fresh feeding signs and small larvae when scouting for aphids/thrips. Addition of a pyrethroid to aphid and thrips sprays can help with looper and earworm control.
Mystery Lepidopterous Larvae Identified: In out last update we mentioned that an unidentified lepidopterous larvae has been found burrowing into the roots of young romaine plants in the Yuma Valley. Because of the unusual coloration and patterns on the larvae, as well as the root boring behavior, we were uncertain of its identity. After rearing the larva to a moth, we were able to identify the Lep species. With the assistance of Ian Watkinson, we have identified the species as a beet armyworm, Spodoptera exigua (see Mystery Larvae Identified). This was surprising given the odd markings and coloration. In fact many (including myself) speculated that it might be a cutworm. But beet armyworm larvae are known to vary in color based on temperature and food source among other things. Why these larvae were boring into the lettuce roots is more confusing. In 24 years of working with beet armyworms in lettuce, I’ve never seen this occur. However, Peter Ellsworth noted that he has occasionally seen them bore into cotton terminals. You learn something new every day.
We are on the final section of virus transmission. Virus transmission by insects is one of the most efficient and economically important transmission in agriculture. When you have insects in your crops, not only you are losing your crops because of feeding/chewing by insects, a lot of insects also act as a vector of plant viruses.
Seven out of 29 orders of insect feeding on living green land plants are vectors of plant viruses.
Insect transmit viruses in 4 distinct modes:
Non persistent transmission: The insects can acquire the virus in a matter if seconds/minutes and they are immediately viruliferous. The virus in retained in the stylet of the insect and are transmitted to the next plant the insect feeds on. The virus is retained in the vector only for few minutes and is lost after insect molting. Most viruses transmitted by aphids are non persistent. So when you see few aphids in your melon field and see cucumber mosaic virus symptoms 1-2 weeks later in your field, don’t be surprised. Aphids are efficient vectors, and since viruses are systemic it takes anywhere from few days to 2-3 weeks for the plants to show symptoms. Thus it is very important to manage insects in the field even if you don’t think the ‘pressure’ is not as high.
Semi-persistent transmission: The insects can acquire the virus in minutes/hours and there is no latent (incubation) period in the insect. The virus can stay in the insects foregut for hours and is lost after insect molting. Some species of aphids and whiteflies fall in this category. Example: Cucurbit yellow stunting disorder virus in melons transmitted by whiteflies.
Persistent circulative: Insects have to feed on virus infected plants for hours/days to acquire the virus and the virus has to incubate for hours/days in the insect. After insect can transmit the virus for weeks. Virus can be present in the vectors hemolymph but there is no multiplication of virus in the insect body. Vectors in this transmission includes: Aphids, leafhopper, whiteflies, treehopper.
Example: Beet curly top virus transmission by beet leafhopper
Persistent propagative: Insects have to feed on virus infected plants for hours/days to acquire the virus and the virus has to incubate for hours/days in the insect. After insect can transmit the virus throughout its lifespan. The virus can multiply in the vector system and often times the virus particles are also passed on to the insect offspring. Tomato spotted wilt virus is transmitted on persistent propagative manner by 9 different species on thrips.
Save the Date : 2024 Plant Pathology Workshop
When: August 29th 8AM-12 PM ( breakfast and Lunch provided by Gowan Company and BASF)
Where: Yuma Ag Center, 6425 W 8th Street
What will covered: Plant Pathology program Updates, past season field trial results (we
have some exciting results to share), Q&A to help better Plant pathology program,
Industry panel discussion for all your industry related questions! See you in few weeks!
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.
Corn earworm:
CEW moth counts increased in most locations over the past 2 weeks; about average for this time of the season.
Beet armyworm:
Trap counts remain low in all locations, and below average for late-January.
Cabbage looper:
Cabbage looper trap counts increased in all locations, but still below average for mid-March.
Diamondback moth:
Adult activity decreased across all locations in the past two weeks. Overall, activity is below average for this time of year.
Whitefly:
Adult movement remained low in all locations consistent with previous seasons.
Thrips:
Thrips adult movement beginning to increase in all locations recently particularly in the Yuma Valley. Activity is well below average for mid-March.
Aphids:
Aphid movement low in most locations but increased slightly in Gila and Dome Valleys. Trap captures well below average for this time of the season.
Leafminers:
Adult activity increased in many areas, about average for this time of season.
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
