Planning Ahead for Whitefly Management on Fall Produce and Melon Crops
As the spring melon harvest begins to wind down it is important to start thinking about whitefly management in fall produce and melons crops. The first line of defense in avoiding whitefly issues in the fall vegetable plantings is for PCAs and growers to be vigilant in their whitefly management program on cotton. In the Yuma area, cotton is the primary host crop for whiteflies during the summer, although alfalfa and sudangrass may serve as alternate hosts in some areas. However, before whitefly management begins in cotton, it is important that whitefly populations be prevented from building up to large numbers in the spring melons that recently finished harvest, or that will be done in the next week or so. In surveying melon crops for CYSDV this spring, it became readily apparent that a large proportion of the spring melon acreage throughout the area was grown near cotton. In fact, our surveys show that on an area-wide basis almost 75% of the melon acreage this spring were grown either adjacent to, or within a 1/2 mile of, cotton. Although whitefly numbers have been relatively light thus far, increased whitefly numbers have been observed over the past week in cotton coinciding with higher temperatures and area-wide melon harvests. Thus, proper sanitation in spring melons is critical for preventing unnecessary whitefly buildups in cotton. It is highly recommended that melon growers quickly destroy plant residue as soon as possible following harvest. A delay in disking under melon fields following harvest can provide a large source of adult whiteflies that can readily disperse into cotton, especially when they don't need to fly very far. These whiteflies may also move into nearby weeds many of which (e.g., common mallow and silverleaf nightshade) are hosts for the Cucurbit Yellows Stunting Disorder Virus (CYSDV). Another source of whiteflies and CYSDV during July and August can be volunteer melons in fields where spring melons had previously been grown. These plants also potentially extend the host acquisition/transmission period for CYSDV. This may be important too since CYSDV incidence in spring melons (albeit at non-economic levels) was quite evident this year. Our experiences to date suggest that the incidence of CYSDV in fall melons is generally much higher in fall plantings growing in proximity to where melons were produced the previous spring. For more information on sanitation practices see Whitefly Management on Desert Vegetable and Melons.
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:
Results of pheromone and sticky trap catches can be viewed here
Corn earworm: Moth activity remains seasonably low, but noticeably increased again in
the North Yuma Valley. Below average compared with previous seasons.
Beet armyworm: Moth activity increasing comparable to fall numbers, particularly in Tacna, Dome Valley and Gila Valley; consistent with reports of larvae in fields. Slightly about average compared with previous seasons.
Cabbage looper: Similarly, cabbage looper moth activity increasing Tacna, Dome Valley
and Gila Valley and remains above average for this time of the year. Above average compared with previous seasons.
Whitefly: Adult movement is relatively absent; typical for this time of the season.
Thrips:Thrips activity remains seasonably low; but is increasing in the Yuma
Valley. Well below average compared with previous seasons
Aphids: Area-wide aphid flights decreased in the last 2 weeks. Most aphids found on traps were green peach aphid. Below average movement compared with previous seasons.
Leafminers: Adult activity remains seasonably low in all locations, and below average for this time of year.
Diamondback moth:
Diamondback moth (DBM) counts in pheromone traps trended downward over the past 2 weeks, and remained low in many locations with the exception of the N. Yuma Valley. Across all locations, trap counts remain higher now than trap counts recorded this time last year, but significantly lower than 2017 following the fall 2016 outbreak.