Seems that insect pests in fall produce crops are like a box of chocolates, you never know what you’re going to get. And this year is particularly interesting. Whitefly pressure has been the lightest I’ve seen in several years. There are exceptions, particularly in areas near alfalfa and where cotton is still being defoliated. Bagrada bug has also been very light this fall, although we are now beginning to pick up a fair number in broccoli plots at the Yuma Ag Center (YAC). Not sure why the numbers are lighter this year, and why they are just now beginning to show up in numbers. I have had reports that they are also showing up in a few areas in Coachella Valley. On the flip side, flea beetle activity has been very heavy this fall and populations remain very active in some locations. At YAC, flea beetle activity remains heavy and can be found on all lettuce and cole crops of all sizes. Worm activity has been erratic this fall, but there are plenty of them around. Beet armyworm pressure was very heavy about 3 weeks ago, subsided a bit last week, and now are coming on heavy again. Egg masses can readily be found on lettuce and broccoli. Perhaps most surprising has been the unusually heavy cabbage looper pressure the past few weeks. We’re averaging 3-4 larvae /plant in many of our efficacy trials, and egg lays remain steady. Have also had reports of diamondback moth on transplants in some areas. We can only find them on low numbers on direct-seeded broccoli. The good news is that our trap counts show that corn earworm moth activity is very low -but definitely don’t forget about them as the head lettuce begins to fold in. With the warm weather expected to continue for the next week or so, I would not anticipate the worm pressure to let up much. Thrips have also been unusually active so far this fall, showing up a few earlier than normal. At the YAC, we’re picking up quite a few bean thrips (see image below) adults. The adults are heavily scarring up the plants, but can be controlled with Radiant and Lannate. It seems like they dispersed into the Yuma Valley following that low pressure system that moved through the area last week. Finally, we’ve been picking up winged-aphids in our trapping network for the past 3 weeks, a bit earlier than normal. I have observed winged cabbage aphid and cowpea aphids on lettuce, and although we’ve yet to find any green peach aphid winged adults, I observed this week a few green peach aphid nymphs colonizing broccoli at YAC. Given that cool weather and windy weather we experienced recently, this is not surprising. Crops treated with imidacloprid should prevent significant colonization on younger crops. But keep your eyes open for aphids on older cole crops, particularly where the imidacloprid residual may be declining.
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: