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 Fusarium Wilt of Lettuce Using Steam Heat – Trial Initiated
Earlier this week, we initiated a trial examining the use of band steam for controlling Fusarium wilt of lettuce. The premise behind this research is to use steam heat to raise soil temperatures to levels sufficient to kill soilborne pathogens. For Fusarium oxysporum f. sp. lactucae, the pathogen which causes Fusarium wilt of lettuce, the required temperature for control is generally taken to be > 140°F for 20 minutes. Soil solarization, where clear plastic is placed over the crop bed during the summer, exploits this concept. The technique raises soil surface temperatures to 150-155˚F, effectively killing the pathogen and reducing disease incidence by 45-98% (Matheron and Porchas, 2010).
In our trials, we are using steam heat to raise soil temperatures. Steam is delivered by a 35 BHP steam generator mounted on a custom designed elongated bed shaper (Fig. 1). Preliminary results were encouraging. The device was able to increase the temperature of the top 3” of soil to over 180°F at a travel speed of 0.5 mph as shown in this video of the machine in action (shown below). These temperatures exceed that of those known to control pathogens responsible for causing Fusarium wilt of lettuce (> 140°F for 20 minutes).
Stay tuned for final trial results and reports on the efficacy of using steam heat to control Fusarium wilt of lettuce.
If you are interested in evaluating the technique on your farm, please contact me. We are seeking additional sites with a known history of Fusarium wilt of lettuce disease incidence to test the efficacy and performance of the device.
References
Matheron, M. E., & Porchas, M. 2010. Evaluation of soil solarization and flooding as management tools for Fusarium wilt of lettuce. Plant Dis. 94:1323-1328.
Acknowledgements
This project is sponsored by USDA-NIFA, the Arizona Specialty Crop Block Grant Program and the Arizona Iceberg Lettuce Research Council. We greatly appreciate their support.
A special thank you is extended to Cory Mellon and Mellon Farms for allowing us to conduct this research on their farm.
Weeds are one of the most visible of all agricultural pests. They can’t move or hide and once established often stick up over the crop. Just one weed in a 10 acre field is annoying to look at. With insects and diseases, the damage is often more visible than the pest. That is not the case with weeds. A moderate weed infestation is approximately 10 weeds per square foot. If a herbicide produces 90% control, that leaves 1 weed per square foot or 43 weeds per acre. Without an untreated check, this can look like the herbicide failed! It is easy to leave an untreated spot in a field and it is well worth doing. Many applicators do so unintentionally because of skips, powerlines and other causes. They help determine crop injury and weed control. Here are some examples of what various levels of control looked like from one of our cole crop trials:
Corn earworm:
CEW moth counts are down areawide average for this time of year.
Beet armyworm:
Trap counts decreased slightly in all locations; about average compared to previous years.
Cabbage looper:
Cabbage looper counts increased in all traps and about average for this time of season.
Diamondback moth:
DBM moths increased significantly in all areas. But still below average for this time of the year.
Whitefly:
Adult movement light and remains below average for this time of year.
Thrips:
Thrips adult counts remain low with the exception of Roll. Currently, below average compared with previous years.
Aphids:
Aphid movement increased significantly in North Yuma and Gila Valleys consistent with storm and winds on Dec 3. About average for early December.
Leafminers:
Adult activity up in a few locations, but below average for December.