This has to be one of the lightest whitefly years we’ve experienced in many years. It started in the spring with lower than normal numbers on melons and continued with low-moderate numbers through the summer on cotton. So far, adult numbers have been unusually low on fall melons, even lower than what we observed at this time last year. For example, whitefly populations sampled in our untreated experimental plots during the last week in August at the Yuma Ag Center, show that adult numbers are the lowest we’ve observed in August since 2007 (Figure 1). Furthermore, numbers recorded from yellow sticky traps placed adjacent to commercial fall melons fields in August and early September in Yuma reveal a similar trend (Figure 2). There are exceptions though. One trap location in Roll has recorded very high whitefly numbers, and the field is equally heavily infested. In this case, it’s no surprise the trap is located about ¼ mile downwind of cotton that is still being irrigated. Nonetheless, you should still plan on whiteflies showing up in produce fields at some point in the next few weeks, particularly in the Yuma Valley where whitefly adults can often appear overnight. It is important that PCAs pay particular attention to early whitefly control on their newly planted produce crops. Prolonged feeding by adult whiteflies on seedling plants can cause stunted plant growth. If you observe honeydew on leaves in the absence of nymphs, then there are way too many adults on the plants. There are likely too many eggs being laid as well. PCAs have options for effective control of adults, and good knockdown can be achieved on lettuce and cole crops with 1) pyrethroids tank-mixed with Orthene, 2) Venom, Assail, or Scorpion (neonicotinoids), 3) Exirel (diamide), and 4) the newly registered Sivanto (buteolide). All of these products will provide 3-7 days or adult knockdown. To assess adult control under heavy migrations, try monitoring young leaves for the presence of light-colored eggs (newly laid) using a hand lens. Absence of newly laid eggs can be an indication that adults are not actively feeding on leaves or are dying before they can lay eggs. Furthermore, allowing adults to remain unchecked on small plants generally results in the development of large nymph populations that can cause significant growth/yield reductions in all produce crops. It is strongly recommended that growers apply a soil insecticide on lettuce and cole crops throughout September, and if the weather remains warm into mid-October. Soil applied imidacloprid (e.g., Admire Pro- 10.5 oz ; Alias 2F-24 oz; Wrangler 4F-12 oz) can provide control through thinning stage. Verimark applied at 13.5 oz/ac can provide excellent control of nymphs when applied at planting similar to the neonicotinoids. Once plants get larger, Movento, Exirel, Venom, Scorpion, Assail, Knack and Courier can provide effective control of nymphs. For more information on whitefly biology, management and insecticide alternatives see these reports: Insect Management on Desert Vegetables and Melons: Whitefly. and Whitefly Management in Fall Produce- 2016
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: