To my surprise, the diamondback moth (DBM) has already returned to the desert, and its timely occurrence happens to coincide with the establishment of early brassica transplants. I was not anticipating this early of an arrival; but they are definitely here. We began to pick up a few DBM moths in traps during the week of Aug 19-26. The last moths caught prior to this was in early July near a brassica seed crop. Then during the week of Aug 26-Sep 2, traps captured a higher number of diamondback moth adults in several areas. A total of 22 DBM adults were caught in 7 traps during that week. May not sound like a lot, but more than what I expected. In all but one location, the moths were caught in traps located adjacent to newly transplanted cauliflower or cabbage fields (See DBM Trap Network). Moreover, since Monday we have seen a sharp increase in moths captured, particularly in Dome Valley and Wellton. At one trap location in Wellton, I counted 31 moths captured over 2 nights. There have also been a number of reports from PCAs in the past 2 days of adult DBM flying within fields. The interesting thing is that no one has reported any eggs, larvae or feeding damage on transplants in the fields where these adults are being found. Could be that the Verimark tray drenches are preventing DBM colonizarition so far. Time will tell. This early moth activity seems unusual to me, but maybe it’s because I’ve been looking so hard for them. The key question is where did these adults come from? The answer is important as it may indicate whether we are dealing with the same diamide resistant population we battled in 2016-17, or a completely different population with resistance to some other chemistry or nothing at all. In my view, there are 3 potential points of origin for these DBM adults. 1) Local Residents - I’ve always assumed DBM would not be capable of spending the summer (mid-June to mid-August) in the desert because of the lack of a suitable host. Our trapping data appears to support this hypothesis since we caught no moths during this period. But don’t know for sure. 2) Hitchhikers – another potential source could be the transplants themselves. Very possible, and can’t be ruled out, but the transplants where moths have been reported/captured have originated from six different nurseries so far (4 from coastal CA and 2 local). Have not picked up any DBM adults in direct seeded broccoli yet, but time may tell. 3) Immigrants - We know that DBM are capable of migrating long distances in winds, and given the widespread occurrences of the moths so far, it may be possible that recent storms may be bringing some of them into the area from the south. It may just be a coincidence that the large increase in moth activity in the last 2-3 days follows a tropical storm disturbance that moved through the area this pest weekend? We may never know the origin, but trust me we will continue to investigate. The bottom line: PCAs and growers should anticipate an early occurrence of DBM this season and prepare accordingly. For more information of managing DBM on fall crops see Guidelines for Diamondback Moth Management in Fall Cole Crops.
In response to the recent outbreaks of Diamondback moth (DBM) , Plutella xylostella in Yuma, we have established a pheromone trap network designed to monitor the activity and movement of adult populations of DBM. PCAs have had difficulty controlling DBM in cabbage, broccoli and cauliflower since October. Traps have been placed in Roll, Wellton, Dome Valley, Gila Valley and Yuma Valley in locations where cole crops are presently being grown or in areas where infestations were known to occur this fall.
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.
In 2022, we had 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. It is always a good idea to look out for any potential diseases that might occur this year!
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.
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.
Corn earworm:
CEW moth counts continue to be active, about average for this time of the season; particularly active in Wellton and Yuma Valley.
Beet armyworm:
Trap counts increased slightly in most locations, but well below average for early October. Most activity in Dome and Yuma Valley.
Cabbage looper:
Cabbage looper trap counts remain steady in most areas. Activity about normal compared to previous years.
Whitefly:
Adult movement increasing Dome Valley and Wellton, but down in many other locations; movement is below average for this time of year, but should anticipate increase movement with melon harvest beginning.
Thrips:
Thrips adult movement increased slightly in some locations, particularly in Wellton and Tacna, but remains below average for mid-September.
Aphids:
Aphid movement has been absent since early June typical with high summer temperatures, and the monsoon flow. Picked a up a few aphids last week with the break in weather and change in wind direction.
Leafminers:
Adult activity remains low in most location; below average for this time of season.
Diamondback moth:
Significant increase in moth activity in the last week, particularly in Dome Valley and N. Gila Valley in traps located adjacent to cauliflower and cabbage transplants.