Seems that insect pests in fall produce crops are like a box of chocolates, you never know what you’re going to get. And this year is particularly interesting. Whitefly pressure has been the lightest I’ve seen in several years. There are exceptions, particularly in areas near alfalfa and where cotton is still being defoliated. Bagrada bug has also been very light this fall, although we are now beginning to pick up a fair number in broccoli plots at the Yuma Ag Center (YAC). Not sure why the numbers are lighter this year, and why they are just now beginning to show up in numbers. I have had reports that they are also showing up in a few areas in Coachella Valley. On the flip side, flea beetle activity has been very heavy this fall and populations remain very active in some locations. At YAC, flea beetle activity remains heavy and can be found on all lettuce and cole crops of all sizes. Worm activity has been erratic this fall, but there are plenty of them around. Beet armyworm pressure was very heavy about 3 weeks ago, subsided a bit last week, and now are coming on heavy again. Egg masses can readily be found on lettuce and broccoli. Perhaps most surprising has been the unusually heavy cabbage looper pressure the past few weeks. We’re averaging 3-4 larvae /plant in many of our efficacy trials, and egg lays remain steady. Have also had reports of diamondback moth on transplants in some areas. We can only find them on low numbers on direct-seeded broccoli. The good news is that our trap counts show that corn earworm moth activity is very low -but definitely don’t forget about them as the head lettuce begins to fold in. With the warm weather expected to continue for the next week or so, I would not anticipate the worm pressure to let up much. Thrips have also been unusually active so far this fall, showing up a few earlier than normal. At the YAC, we’re picking up quite a few bean thrips (see image below) adults. The adults are heavily scarring up the plants, but can be controlled with Radiant and Lannate. It seems like they dispersed into the Yuma Valley following that low pressure system that moved through the area last week. Finally, we’ve been picking up winged-aphids in our trapping network for the past 3 weeks, a bit earlier than normal. I have observed winged cabbage aphid and cowpea aphids on lettuce, and although we’ve yet to find any green peach aphid winged adults, I observed this week a few green peach aphid nymphs colonizing broccoli at YAC. Given that cool weather and windy weather we experienced recently, this is not surprising. Crops treated with imidacloprid should prevent significant colonization on younger crops. But keep your eyes open for aphids on older cole crops, particularly where the imidacloprid residual may be declining.
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:
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
