Now that produce planting/transplanting is well underway, one of the first pests you’re likely to encounter is adult whiteflies. Given the high numbers found on spring melons and cotton this summer, one would also expect whiteflies to be heavy on fall produce crops. Surprisingly though, adult numbers have been lighter than expected on fall melons. For instance here at the Yuma Ag Center, whitefly populations in our experimental plots based on vacuum samples taken this week are lower than what we’ve encountered the past few years (see graph below). However, this appears to be site-specific as I’ve observed melon fields in the Wellton/Roll area with very high numbers this past week. This makes sense; whitefly population abundance can be influenced to a large extent by the crop landscape throughout Yuma. Crops planted near or adjacent to cotton and alfalfa are clearly more heavily infested than fields isolated away from these crops. In addition, our monsoon conditions often play a role where humidity and rainfall can influence population abundance. Nonetheless, you can bet heavy whitefly numbers will eventually show up in one of your fields. It is important that PCAs pay particular attention to early whitefly control on their newly planted produce crops. Prolonged feeding by heavy numbers of adults on seedling lettuce/cole crop plants can cause stunted plant growth. If you observe honeydew on leaves in the absence of nymphs then there are way too many adults on the seedling plants. There are likely too many eggs being laid as well. With the loss of endosulfan, growers have few options for effective residual control of adults, but good knockdown can be achieved on lettuce and cole crops with combinations of pyrethroids tank-mixed with Orthene, Lannate, Lorsban, Venom, Scorpion, or Sequoia. In addition, Exirel (the foliar formulations of Cyazypyr) was recently labeled for use on produce crops and can provide excellent adult control. Sometimes it can be difficult to determine whether a product is working 3-4 days following application when adult whiteflies are continuously moving into you field from outside sources. To assess adult control under heavy migrations, try monitoring young leaves for the presence of light-colored eggs (newly laid) using a hand lens. Absence of newly laid eggs can be an indication that adults are not actively feeding on leaves or are dying before they can lay eggs. Furthermore, allowing adults to remain unchecked on small plants generally results in the development of large nymph populations that can cause significant growth/yield reductions in all produce crops. It is strongly recommended that growers apply a soil insecticide on lettuce and cole crops throughout September and early October. Local research has shown that imidacloprid applied at 0.25 lbs AI/ac at planting provides less residual control of nymphs today than it did 10 years ago. However, given the current economics of imidacloprid, cost-effective whitefly control can still be achieved by using higher rates of imidacloprid (0.38 lbs. AI/ac) to extend residual control (e.g., Alias 2F-24 oz; Wrangler 4F-12 oz; Admire Pro- 10.5 oz). Verimark, the soil formulation of Cyazypyr, applied at 13.5 oz/ac can provide excellent control of nymphs when applied at planting similar to the neonicotinoids to lettuce and cole crops. Once plants get larger, products like Movento, Exirel, Venom, Scorpion, Assail, Knack and Courier/Vetica can provide effective control of nymphs. For more information on whitefly biology, management and insecticide alternatives see these reports: Management in Fall Produce- 2014 and Insect Management on Desert Vegetables and Melons: Whitefly.
We are on the final section of virus transmission. Virus transmission by insects is one of the most efficient and economically important transmission in agriculture. When you have insects in your crops, not only you are losing your crops because of feeding/chewing by insects, a lot of insects also act as a vector of plant viruses.
Seven out of 29 orders of insect feeding on living green land plants are vectors of plant viruses.
Insect transmit viruses in 4 distinct modes:
Non persistent transmission: The insects can acquire the virus in a matter if seconds/minutes and they are immediately viruliferous. The virus in retained in the stylet of the insect and are transmitted to the next plant the insect feeds on. The virus is retained in the vector only for few minutes and is lost after insect molting. Most viruses transmitted by aphids are non persistent. So when you see few aphids in your melon field and see cucumber mosaic virus symptoms 1-2 weeks later in your field, don’t be surprised. Aphids are efficient vectors, and since viruses are systemic it takes anywhere from few days to 2-3 weeks for the plants to show symptoms. Thus it is very important to manage insects in the field even if you don’t think the ‘pressure’ is not as high.
Semi-persistent transmission: The insects can acquire the virus in minutes/hours and there is no latent (incubation) period in the insect. The virus can stay in the insects foregut for hours and is lost after insect molting. Some species of aphids and whiteflies fall in this category. Example: Cucurbit yellow stunting disorder virus in melons transmitted by whiteflies.
Persistent circulative: Insects have to feed on virus infected plants for hours/days to acquire the virus and the virus has to incubate for hours/days in the insect. After insect can transmit the virus for weeks. Virus can be present in the vectors hemolymph but there is no multiplication of virus in the insect body. Vectors in this transmission includes: Aphids, leafhopper, whiteflies, treehopper.
Example: Beet curly top virus transmission by beet leafhopper
Persistent propagative: Insects have to feed on virus infected plants for hours/days to acquire the virus and the virus has to incubate for hours/days in the insect. After insect can transmit the virus throughout its lifespan. The virus can multiply in the vector system and often times the virus particles are also passed on to the insect offspring. Tomato spotted wilt virus is transmitted on persistent propagative manner by 9 different species on thrips.
Save the Date : 2024 Plant Pathology Workshop
When: August 29th 8AM-12 PM ( breakfast and Lunch provided by Gowan Company and BASF)
Where: Yuma Ag Center, 6425 W 8th Street
What will covered: Plant Pathology program Updates, past season field trial results (we
have some exciting results to share), Q&A to help better Plant pathology program,
Industry panel discussion for all your industry related questions! See you in few weeks!
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 remain low across all locations; average for this time of the season.
Beet armyworm:
Trap counts decreased in all locations, and below average for late-January.
Cabbage looper:
Cabbage looper trap counts remained low in all locations; below average for early February. Diamondback moth:
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.
Whitefly:
Adult movement remained low in all locations consistent with previous seasons.
Thrips:
Thrips adult movement beginning to increase slightly in most locations. Activity a about average for mid-February.
Aphids:
Aphid movement increased in many locations, particularly in Yuma Valley. Trap captures about average for this time of year.
Leafminers:
Adult activity increased in most areas, below average for this time of season.