Pest pressure in desert produce has begun to subside, but don’t become complacent. The local weather forecast calls for above average temperatures for the next 10 days and accordingly, could extend pest activity, particularly in crops near harvest. Below are some thoughts on the status of local pest pressure based on field sampling, trap counts and PCA reports. Beet armyworm
Populations have been down for a couple of weeks, and spray intervals have lengthened. This isn’t surprising given the cooler tempers we experienced in early October and the intensive spraying during September that provided areawide suppression. But they are still out there. Cabbage looper
Loopers were by far the most abundant Lepidopterous pest I saw this fall, but their numbers have noticeably dropped in the past 2 weeks. Be cautious though. With warm weather forecast for next week they could surprise you, especially on produce approaching harvest. Similarly, don’t relax with Corn earworms. Although trap captures have been down the last few weeks and no recent reports of larvae in head lettuce, it only takes a couple of these dudes at harvest to ruin your day. (See Corn Earworm on Desert Producefor more information) Diamondback moth
Trap counts have been below average this season, but we’re seeing an abundance of larvae on the Ag Center and have had numerous reports from PCAs treating for larvae in both direct-seeded broccoli and transplanted cabbage and cauliflower. Highest trap catches have been in traps adjacent to newly established seed crops and organic transplants (see Areawide DBM Trap Network). It is advisable to closely examine your earliest plantings a little more closely for the next few weeks, looking for feeding signs in terminal leaves. Remember, larvae hatch on outer leaves but quickly migrate to the growing point. They also do better than other worms in cooler weather (see 2021 Guidelines for DBM Management in Desert Cole Cropsfor more information). Thrips
Western flower thrips populations have slowly been building on romaine at YAC, and areawide traps indicate adult movement has slowly increased in the past week. Because of the potential for INSV to move into the desert on coastal transplants, it is advised that PCAs keep thrips populations suppressed as much as possible in early fields (particularly transplanted lettuce originating from Salinas). It might be worth making an extra spray just to prevent secondary spread to adjacent fields. Also, stress to your growers to disc under recently harvested lettuce/spinach field ASAP. Remember the key to thrips control is to maintain larval populations at low levels. The cryptic or thigmotactic behavior of thrips often makes them difficult to find on lettuce plants. So, when scouting if you find a few, there are probably a lot more that you don’t see within the plant. Aphids
Reports of aphids colonizing brassica transplants are starting to come in. They likely hitched a ride on the transplants and may soon be spreading to adjacent fields. As anticipated, we’re beginning to pick up green peach aphids on our sticky traps and have numerous reports by PCAs of winged (alate) aphids beginning to show up on plants. Remember, aphids on produce are cool season pests and do best when temps dip below ~ 70° F. Stay vigilant, proper ID is important for the selection of the most cost-effective insecticide. Like thrips, it’s best to “nip aphid colonization in the bud”.
Downy Mildew of Lettuce and 4th annual downy mildew field day
Downy mildew has always been one of the major problem for PCAs and growers in the desert southwest. The symptoms observed are green to yellow angular spots on the upper surface of the leaves and fluffy growth on the lower side (See Picture). Symptoms usually start from older leaves. As disease progresses the lesion turn brown and dry up and in some occasions the disease can become systemic causing dark discoloration of vascular tissue. Favorable condition for disease development:
The pathogen Bremia lactucae thrives in damp, cool condition, with moisture present on leaves. Spores are short-lived but dispersed efficiently by wind during moist period. Cultivated lettuce is the main host of the pathogen but it has also been reported to infectartichoke, cornflower and strawflower. Why is downy mildew difficult to manage?
One of the main reason that hinders the disease management is the complexity of the pathogen. Bremia lactucae consists of multiple races (pathotypes), and new races continue to occur as pathogen evolves. The pathogen is one of the fastest evolving plant pathogen. And each pathotypes have developed insensitivity to fungicides to different extent.
One of the best practice is to grow resistant cultivar, but there are limitations. As the pathogen is highly variable and dynamic, resistant cultivars are not a permanent solution as the pathogen overcomes the resistance by evolving into virulent strains and isolates.
Preventative application of fungicides are effective to some extent. Reducing leaf wetness and humidity by using drip or furrow irrigation can be helpful. However, weather condition like rain during cool weather as we had in past couple of weeks is conducive to development of epidemics and we have very little control on that matter.
4th ANNUAL DOWNY MILDEW FIELD DAY
Save the date for March 8, 10 AM- Noon for 4th annual field day (See attached flyer). 2.0 CA and AZ CEU have been applied for. Taco truck will be there at noon for lunch.
Band-Steam Applicator for Controlling Soilborne Pathogens and Weeds in Lettuce
Steam sterilization of soils is commonly used in plant nurseries and greenhouses for effective control of soilborne pathogens and weed seeds. The technique, however, is highly energy intensive as the entire soil profile is heated. This is too costly and slow to be practical for field scale vegetable production. To reduce energy consumption and cost, use of band-steaming, where steam is applied only in the area where it is needed – in the plant root zone, is proposed. In this method, narrow strips of soil centered on the seed line are treated with steam rather than the whole bed.
Over the course of the last year, we developed a prototype band-steam and co-product applicator that is designed to raise soil temperatures in a band 2” deep by 4” wide to levels sufficient to control soilborne pathogens (140 °F for > 20 minutes) and weed seed (150 °F for > 20 minutes). The device is principally comprised of a 35 BHP steam generator and a co-product applicator mounted on top of a bed shaper (Fig.1). The apparatus applies steam via shank injection and from cone shaped ports on top of the bed shaper. An exothermic compound can be co-applied via shank injection and/or a banding spray nozzle. The rationale behind co-applying an exothermic compound with steam is that exothermic compounds react and release heat when combined with water, thereby reducing energy requirements and increasing travel speed.
Preliminary testing of the device this spring in Yuma, AZ were very promising. Trial results showed that application of steam alone effectively raised soil temperature in the center of the seed line to levels required for effective pest control (140 °F for more than 20 minutes). Use of the exothermic compound increased soil temperature by about 10 °F. A video of the device in action can be found at the link provided below.
We are currently evaluating the device in field trials with lettuce in Salinas, CA. Target pests in these experiments conducted in collaboration with Steve Fennimore, UC Davis, are soil pathogens which cause Sclerotinia lettuce drop and in-row weeds. Future articles will report the findings of this research.
This fall, we will be replicating these tests in Yuma, AZ and also investigating the effectiveness of band-steam for controlling Fusarium oxysporum f. sp. lactucae which causes Fusarium wilt of lettuce. Heat has been shown to effectively kill Fusarium oxysporum spores and control Fusarium wilt disease. As an example, soil solarization, where clear plastic is placed over crop beds during the summer, raises soil temperatures to 150-155˚F at the soil surface, effectively killing the pathogen and reducing disease incidence by 45-98% (Matheron and Porchas, 2010).
These projects are sponsored by USDA-NIFA, the Arizona Specialty Crop Block Grant Program and the Arizona Iceberg Lettuce Research Council. We greatly appreciate their support.
If you are interested in seeing the machine operate or would like more information, please feel free to contact me.
See the band-steam and co-product applicator in action!
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.
Sprangletop has become increasingly widespread in Arizona mostly because of its growth habits and tolerance to many commonly used herbicides. It is in the Leptochloa genus which is derived from the Greek words leptos (thin) and chloa (grass). There are more than 150 species of sprangletop worldwide but only three in Arizona and two in Yuma County. The two that are the most common in the low desert are Mexican Sprangletop, which is Leptochloa uninervia and Red Sprangletop, Leptochloa filiformis. A third species, Bearded Sprangletop, Leptochloa fascicularis, is more common at higher elevations of 1500 feet or higher. It is not uncommon to find both Red and Mexican Sprangletop in the same field and it is not hard to distinguish them when they are side by side. Red Sprangletop has a light green leaf blade which is similar in width to watergrass and barnyardgrass. It has very fine hairs and very small and fine branches and spiklets. It also has a long membranous ligule. The name Red refers to the leaf sheath, which is characteristically red, rather than the seed head. Mexican Sprangletop has a thinner leaf blade which is darker green or grayish in color and similar in appearance to common bermudagrass. The seed head is distinctly coarser than that of Red Sprangletop. Side by side, leaf color and size of the seed make it easy to distinguish these two. Both of these grasses are classified as summer annuals, but they grow more like perennials in the low desert. Sprangletop does very well in the hottest part of the summer and typically germinates from seed during the hottest period between July and September. Once established, however, it often survives through the cold winter months. It grows into clumps that often appear to be dead during the winter. New shoots commonly grow from these established crowns the next season. When this occurs, preemergent herbicides such as Trifluralin or Prowl are ineffective. Some Sprangletop plants stay green and grow through the winter. Many of the postemergence, grass specific herbicides that control many grasses are ineffective on Sprangletop. This also has contributed to the spread of these weeds. Sethoxydim (Poast) and Fluazifop (Fusilade) do not control either Red or Mexican sprangletop. Only Clethodim (Select Max, Select, Arrow and others) is the only one of these grass herbicides that is effective and only at the highest labeled rates. Two applications are often necessary to achieve season long control.
