We all know that weather influences produce crop growth and ultimately can create wide fluctuations in markets. Similarly, our local weather can significantly impact pest activity on desert produce crops. For insects, their activity and abundance are closely aligned with weather. The reason for this is that temperature is the driving force behind insect development, growth and behavior. Unlike many animals, insects are poikilothermic (“cold-blooded”); that is, they are unable to regulate their body temperature. Their internal temperature varies along with that of the ambient environmental temperature. Consequently, insect pests such as whiteflies, beet armyworms, cabbage loopers, diamondback moths, and leafminers are more active and develop rapidly when temperatures average 85-90 F; in contrast, they are less active and develop much slower under cool, winter conditions. That is one of the primary reasons these insect pests can be so abundant on produce crops in Sep-Nov. Behavioral activities such as flight, movement, reproduction, feeding and oviposition are similarly influenced by seasonal temperatures. But temperature requirements for insects vary with species (see table below). For example, the optimal temperature for growth and development of beet armyworms and diamondback moths is the same (86 °F), but diamondback moth can complete a generation quicker and can develop over a much broader range of temperature. Thus, they can be more problematic season long if not adequately controlled. Then you have Bagrada bug which tend to prefer warmer weather for development. Its lower developmental threshold of 62°F is the main reason the pest is generally hard to find during the winter months. In contrast, green peach aphids, which we annually find on winter produce crops, thrive in cooler weather (optimal temperature for growth is 55°F. That’s the main reason we don’t find them on summer crops; it is just too hot for them. In general, extreme hot (> 120°F) or cold temperatures (<32 °F), can greatly restrict insect growth and behavior, and can be lethal. Consequently, temperature and other weather factor such as rainfall, humidity, sunlight and wind play a major role in determining insect activity and abundance on local desert crops; but they can also suppress insect abundance under extreme conditions. For a more detailed explanation on the impact of weather on insects visit Weather Influences Desert Insect Pests. And if you have time, join us on December 19th for what will undoubtedly be an interesting meeting, weather permitting.
Bacterial Fruit Blotch (BFB) in melon seedlings (2024)
Bacterial fruit blotch (BFB) of melon is caused by the bacterium Acidovorax avenae subsp. Citrulli. The bacteria produces large olive green to brown water-soaked lesions on fruit, making them unmarketable. Symptoms of BFB on seedlings begin with water-soaked areas on the lower surface of the cotyledons and inconspicuous lesions on leaves. BFB lesions will become necrotic often with yellow halos. Lesions are frequently delimited by veins. Infected seedlings collapse and die. Greenhouse conditions are usually favorable for dispersal and establishment of pathogen. Thus, good greenhouse practices and sanitation is extremely important. Clean transplant trays must be used (disinfect trays if they will be reused) and new soil. Destroy any volunteer seedlings and keep the area in and around the greenhouse weed free. Avoid overhead watering if at all possible, or water in the middle of the day so that the plants dry thoroughly before evening. The bacterium can spread on mist and aerosols. Relative humidity should be kept low through proper watering and good air circulation in the greenhouse. Separate different seedlots, to reduce lot-to-lot spread. Monitor these isolated seedlings daily and destroy trays where symptoms develop. The remaining trays should be sprayed with a labeled bactericide and the applications continued until the plants are transplanted to the field. The pathogen can be seedborne, so growers should only use seed that has been tested for the presence of the pathogen by a reputable testing facility. Management of BFB includes a combination of preventing the introduction of the pathogen, sanitation to eliminate any inoculum present, and the use of bactericides if the disease appears. There are no commercially available watermelon cultivars that are resistant to bacterial fruit blotch, but there is some variation in susceptibility among cultivars.
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: