Over the past two weeks, the sticky traps in our Areawide Insect Trapping Network placed near recently disked up produce fields have been literally covered with flies. Many of the flies identified on these traps were adult seedcorn maggot (SCM). This is not surprising since it is this time of the year that SCM can cause significant stand reductions in spring melons and other large seeded crops due to larvae feeding on germinating seed, roots and even stems. If SCM populations are high, replanting parts or all of an infested field is often necessary. Not only is this an inconvenience to the grower, but replanting is expensive and can disrupt harvest schedules. Unfortunately, once maggots have been found infesting the soil during stand establishment, there is usually nothing one can do. Thus, avoidance of the problem is the most effective way of preventing stand reductions. First, weather plays a major role in determining the damage potential for SCM to be a problem. Melon stands are more susceptible to SCM during wet, cool spring weather in which seed germination is slowed or delayed. These conditions give SCM a chance to develop in the soil and attack the seeds before they can emerge. So far this spring, our weather conditions have been just the opposite, warm and dry. Secondly, our cropping system plays a key role. Melon crops following produce are the most often attacked because SCM are attracted to fields with high levels of decomposing organic matter. This includes heavy plant residue remaining after harvest of the previous lettuce or cole crop, as well as applications of manure prior to planting. Growers would be encouraged not to plant melons into fields under these conditions. Unfortunately, given the demands on local acreage, this is sometimes not practical. However if growers decide to plant in these conditions, then it would be wise to use a preventative insecticide applied at planting to minimize the impact from SCM and give seedling stands a fighting chance. Guidelines for SCM management can be found in Seedcorn maggot 2014, and alternatives that have shown activity against SCM and may be practical for SCM management in spring melons can be found in SCM Control on Cantaloupes.
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:
Corn earworm:
CEW moth counts remained low over the past 2 weeks across all locations and about average for this time of the season.
Beet armyworm:
Trap counts decreased in most locations, and well below average for late-November. Most activity in Yuma Valley.
Cabbage looper:
Cabbage looper trap counts remained low in most areas but increased in the Yuma Valley. Activity below average for late November.
Diamondback moth:
Adults peaked in Bard, Gila and Yuma Valleys and slightly above average for this time of year. Traps located adjacent to cauliflower seed crops had the highest trap captures
Whitefly:
Adults remains active in Dome Valley and Roll consistent with melon crops completing harvest, but below average movement for this time of season.
Thrips:
Thrips adult movement decreased in most locations last week, and most active in Dome Valley, Wellton, and Tacna. Activity about average for mid-November.
Aphids:
Aphid movement peaked so far this season with highest activity in Dome Valley, Bard, N. Yuma Valley and N. Gila Valleys over the past 2 weeks. Activity average for this time of year.
Leafminers:
Adult activity increased sharply in the Dome, Yuma, and Gila Valleys, about average for this time of season.