Lettuce aphids, Nasonovia ribisnigri, (aka., red aphid) have been found colonizing lettuce fields since mid-December in several locations throughout Yuma. In the desert, we typically associate lettuce aphid with the warm growing conditions in February and March. However, in the past few weeks, I've identified lettuce aphids infesting head and romaine lettuce in both organic and conventional production in Dome Valley, Gila Valley, Yuma Valley and across the river in Mexico. This is by far the heaviest I’ve seen lettuce aphids colonize lettuce in the desert this early in the season. Foxglove aphid is also being reported in the Yuma Valley, albeit at much lower abundance. Given their widespread incidence, Lettuce aphids likely came into the desert on one of the multiple flights of winged aphids we’ve reported from sticky traps this winter (see Areawide Trapping Network). Experience has taught us high temperatures in the 70-80's are ideal for population growth of these aphid species. Our local weather forecast suggests that high temperatures will be in the 70’s with moderate nighttime lows in the 50’s for the next two weeks. Assuming the weather service is correct, these conditions may be conducive for lettuce and foxglove aphid population growth. So be prepared to battle them.
A few things to remember about lettuce aphids relative to the other aphids we commonly see. First, the immature nymphs are small and have a red appearance, whereas the apterous (wing-less) adults are typically a large brown colored aphid with dark bands running across the abdomen (see images below). Second, among the crops we grow locally, lettuce aphids are only found on the lettuce types (Lactuca spp.). Lettuce aphids prefer to colonize the terminal growth, and can often be found in heads or hearts, whereas green peach aphids are often found on the frame leaves in high numbers before moving into the heads. Sampling should be focused on the terminal growth of young plants, and in the heads and hearts of older plants. Third, they can reproduce prolifically, producing many more alates (winged) adults than other species, which can quickly lead to widespread abundance in a field and dispersal throughout a growing area. Finally, as the saying goes "he who hesitates is lost". If lettuce aphids are found on your lettuce, it is recommended that you respond quickly with an insecticide treatment. The product of choice is Movento at 5 oz/ac. Because of its systemic activity, Movento will reach aphids in the protected terminal growth. Be sure to include a penetrating adjuvant for best results at a rate of at least 0.25% v/v. It normally requires about 7 days of activity before significant reduction in the infestation is observed but can take longer (10-14 days) in cool, cloudy weather. Sequoia and Sivanto are good rotational products if coverage of terminal areas can be attained. For more information on lettuce aphid please refer to Lettuce Aphid on Spring Produce-2022. If you’re finding lettuce aphid on organic lettuce, good luck; we’ve not had much success in controlling them with available biopesticides.
Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum, is one of the oldest described Fusarium wilt diseases and the most economically important disease of watermelon worldwide. It occurs on every continent except Antarctica and new races of the pathogen continue to impact production in many areas around the world. Long-term survival of the pathogen in the soil and the evolution of new races make management of Fusarium wilt difficult.
This year we have a lot of watermelon fields infected with Fusarium from Winterhaven to Yuma, Wellton, and Mohawk Valley. Rain, and overwatering of fields when plants set fruits might have contributed to the disease development.
Symptoms of Fusarium can sometimes be confused with water deficiency, even though there is plenty of water in the field. In Yuma valley we have seen fusarium problem in some overwatered fields.
Initial symptoms often include a dull, gray green appearance of leaves that precedes a loss of turgor pressure and wilting. Wilting is followed by a yellowing of the leaves and finally necrosis. The wilting generally starts with the older leaves and progresses to the younger foliage. Under conditions of high inoculum density or a very susceptible host, the entire plant may wilt and die within a short time. Affected plants that do not die are often stunted and have considerably reduced yields. Under high inoculum pressure, seedlings may damp off as they emerge from the soil.
Initial infection of seedlings usually occurs from chlamydospores (resting structure) that have overwintered in the soil. Chlamydospores germinate and produce infection hyphae that penetrate the root cortex, often where the lateral roots emerge. Infection may be enhanced by wounds or damage to the roots. The fungus colonizes the root cortex and soon invades the xylem tissue, where it produces more mycelia and microconidia. Consequently, the fungus becomes systemic and often can be isolated from tissue well away from the roots. The vascular damage we see in the roots is the defense mechanism of the plant to impede the movement of pathogen.
Disease management include planting clean seeds/transplants, use of resistant cultivars, crop rotation, soil fumigation, soil solarization, grafting, biological control. An integrated approach utilizing two or more methods is required for successful disease management.
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.
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.
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.
Managing Water with Preemergence Herbicides in Lettuce
We are always adjusting how we use herbicides to fit the unique conditions in this area. The herbicides that are registered for use on lettuce here are limited and they all require a little different management. Environmental conditions, soil characteristics and Chemical properties all can greatly affect how well the 3 preemergence herbicides used in lettuce will work. These include Balan, Prefar and Kerb. Environmental conditions and soil characteristic vary greatly from year to year and field to field. It is difficult to make general recommendations on how best to use these three herbicides because of this. Chemical characteristics do not vary, however, and we can make some generalizations on how they should be used.
We use a lot of water here during stand establishment and at this time of year. The water solubility of Balan, Prefar and Kerb vary widely and should be considered when deciding how to use them. Water solubility is the amount of the herbicide that will dissolve in water. This is usually given as PPM or mg/liter. The higher the number the more soluble it is. Solubility will effect leaching into the soil and runoff. The solubility of Balan is 0.1 PPM,Prefar is 5.6 PPM and Kerb is 15 PPM. What this means is that Balan is very insoluble and has to be mechanically incorporated. Prefar is 56 times more soluble than Balan and can be incorporated with overhead water but this is still not a very soluble herbicide and a lot of water is needed. Kerb is 150 times more soluble than Balan and almost 3 times more soluble than Prefar. Kerb will leach and the amount of overhead water applied must be carefully managed.
CEW moth counts remain low across all locations; average for this time of the season.
Trap counts decreased in all locations, and below average for late-January.
Cabbage looper trap counts remained low in all locations; below average for early February.
Adult activity increased slightly in some locations, particularly fields where trap is adjacent to with nearby brassica seed crops. Overall, activity is below average for this time of year.
Adult movement remained low in all locations consistent with previous seasons.
Thrips adult movement beginning to increase slightly in most locations. Activity a about average for mid-February.
Aphid movement increased in many locations, particularly in Yuma Valley. Trap captures about average for this time of year.
Adult activity increased in most areas, below average for this time of season.