Now that the days are getting longer and the temperatures a bit warmer, aphids are likely to be more common. Experience has also shown us that aphid population growth begins to increase significantly once the weather warms in January. Our local weather forecast suggests that temperatures will be in the upper 60’s with moderate nighttime lows in the mid-40’s for the next two weeks. Assuming the weather service is correct, these conditions may be ideal for aphid population growth. Similarly, with the rainfall we experienced in December, I would keep an eye for aphid pressure to start picking up on your lettuce, celery, spinach and cole crops. Previous research has shown us that aphids tend to be more abundant during mild, wet winters.
So far, aphid appear to be ahead of schedule. Aphid flights peaked in mid-December which is a little later than what is expected (see graph below). There have been several reports of aphids showing up in both conventional and organic produce in the past 2 weeks, and at the Yuma Ag center, we have found enough green peach aphids colonizing small lettuce to initiate efficacy trials. Consequently, PCAs should be extra vigilant in scouting for aphid colonies. Be on the lookout for foxglove aphid too; they tend to be more biologically active under warmer temperatures (see Aphid ID Guide). If a field has been treated with imidacloprid at planting, then chances are you are in pretty good shape on young produce. However, imidacloprid doesn’t last all season. Regardless of whether you find green peach or foxglove aphid, the key to effective aphid management with foliar insecticides is to initiate sprays at the time apterous (wingless) aphids begin to colonize. The action threshold is the presence of aphid colonies on 5-10% of plants sampled. Of course, this requires diligent scouting and sampling. On older lettuce, make sure you thoroughly examine developing heads/hearts. Fortunately, PCAs have many alternatives for aphid control (see Aphid Management on Desert Produce_2020). So, keep your eyes open for new aphid colonies and treat early to keep ahead of the populations. For more information on aphid management see: Keys to Aphid Management in Leafy Vegetables.
With the cauliflower/broccoli/cabbage season, the black rot is a common disease in those plants. In Yuma area black rot has been seen in early as well as late stage on plants. In early season the disease is more of a concern in the greenhouse, and later in the season we see the disease in the field. Black rot is a disease in cole crops caused by bacterium, Xanthomonas campestris pv. Campestris.
The pathogen can enter the field on transplants or seeds, and can stay in the soil for over a year where it survives on debris and weeds in the Brassicaceae family. The bacteria move primarily with water and can enter the plant through pores naturally present on the leaf surface. It can also be spread by workers and equipment moving through the field/greenhouse, and can enter the leaves through damage caused by wind, insects and other mechanical injuries. The bacteria need a temperature of 77 degrees and above. While it may seem like the weather in desert is not humid enough for bacteria to become an epidemic, it is not something that should be ignored. The microclimate in the field with the crop canopy being dense, foggy mornings, increased field activity can be conducible for disease development.
Disease management include getting rid of infected plants/tissue, increasing distance between fields and choosing well-drained, controlling weeds especially in mustard family such shepherd’s purse, wild mustard, pepper weed etc. Soil fumigation is effective but may not be economically feasible. Best management practice starts with disease-free seed; hot water treatment can reduce the likelihood that the bacteria will enter the field through the seed. Starting with healthy transplant is equally important. For diagnosis, you can bring the samples to the clinic or you can also do visual/field diagnosos. You will see patches of dead tissue in plants with blackened veins. Usually when the disease is systemic, you can see the black streak when you cut cross section of the stem (see picture below).
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: