Proper identification of winged aphid species found on leafy vegetables in the desert is important for cost-effective pest management. This is the time of the growing season when we often observe winged (alate) aphids appearing on desert lettuce and cole crops. Most of the important aphid species we find on local crops do not over-summer here because of high temperatures. Thus they typically begin migrating onto desert crops beginning in November, often being blown in with the gusting winds. My experience over the past 20 years suggests this is due in part to cooler weather and changes in prevailing winds that now begin to blow into the area from the north and north/west. Consequently, once the aphids reach our desert valleys, they typically move from crop to crop until they find a suitable host to feed and colonize on. It is not uncommon to find winged aphids on lettuce or broccoli that are specific pests of small grains (i.e., corn leaf aphid) or alfalfa (i.e., pea aphid). Because these aphid species will not colonize lettuce, it is important to be able to distinguish them from the key aphid pests commonly found on lettuce that do colonize and require management to prevent problems at harvest (green peach aphid, foxglove aphid, lettuce aphid). Also, you are likely to find cowpea aphid in lettuce as it is common in alfalfa at this time. However, experience has shown us that although small colonies may be found on lettuce, the populations rarely increase in lettuce crops. The bottom line: proper aphid identification can save a PCA time and money, and prevent unnecessary insecticide applications. A simple pictorial key provides information that can assist PCAs with identifying winged aphids important in lettuce and other leafy vegetables. Please click on the following link to view the Aphid ID tool.
Alfalfa is a major crop grown in Yuma area as a cover crop before vegetable season. Generally expected to be a hardy and easy crop to grow, alfalfa production still can be affected by many pests and diseases. Root and crown rot in alfalfa is a common problem. The pathogen Phytopthora megasperma thrives on standing water, overwatered, and/or in soil that lacks good drainage.
The above ground symptoms typically consists of plant wilting, usually occurring quite suddenly, often followed by plant death. The roots turn tan-to-brown, sometimes turning black. Sometimes orange-to-reddish streaks can be observed in rotted roots. Occasionally, the symptoms can be seen in crowns too. Infected plants succumb to death, or grow at reduced rate thus being susceptible to other pests and diseases.
The causal organism survives in soil as mycelia in infected plant tissue or as thick-walled oospores. It also produces thin-walled sporangia that release motile zoospores in the presence of free water. So it is important to grow alfalfa in well drained soils. Adjust irrigation water in a way that there is no standing water for too long. If possible, install tailwater ditch to remove excess water. Plant resistant cultivars like Agate, Apollo if the problem persists in the field. Till the soil deeply to reduce wet soil pockets, level the land before planting.
If diagnosis is confusing, bring the samples to the Yuma Plant Health Clinic for proper diagnosis. Make sure to bring healthy looking plants, sick plant, plants in between etc. Remember, there is no such thing as too much plant tissue when it comes to submitting samples to the clinic!
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:
Area wide Insect Trapping Network VegIPM Update, Vol. 11, No. 22, October 28, 2020
Results of pheromone and sticky trap catches can be viewedhere.
Corn earworm:Moth activity is about average for mid-October, particularly in Dome Valley and south Yuma Valley.
Beet armyworm: Moths remain active throughout the desert, especially active in Tacna and Dome Valley.
Cabbage looper: Cabbage looper activity remains unusually low for early October, likely a result of unusually hot weather. Larvae are yet to show up in many fields.
Whitefly: Adult movement has been about average for this time of year. Activity highest in Tacna near fall melons.
Thrips: To date, thrips activity has been seasonably low at all trap locations; activity increased significantly in Roll.
Aphids: Aphids beginning to show in most traps along the Colorado River (Bard, Gila, Yuma) which is normal for this time of year. Recent high winds may begin to disperse them throughout the area.
Leafminers: Adult activity below normal for September, but high numbers caught in Wellton in areas where melon harvest has commenced.