With the rain we experienced over the weekend, you may likely see a decline in thrips numbers on leafy vegetables. Precipitation amounts varied throughout the area with the highest recorded AZMET levels being recorded in the Gila Valley at 0.46", and the least being recorded at the Roll station (0.14 "). The Yuma Valley received from about 0.40" of rainfall on Saturday. However, as we saw last year with much heavier and more frequent rainfall, these declines can be short-lived. Typically, heavy rainfall with dislodge or even drown adult thrips on plants, and can suffocate larvae in the soil if it remains wet for prolonged periods. Once the plants dry though adults can quickly move back onto plants. Another factor that PCAs should be concerned with this time of the year is thrips "bioconcentration" which occurs each year in late February and March as lettuce acreage declines. This is coincident with an increase in thrips abundance that occurs due to higher temperatures. Each time a lettuce field is harvested and disked, adult thrips populations disperse from these areas into the next available lettuce field. As the number of lettuce acres become reduced at the end of the season, this creates a bottleneck effect that concentrates high numbers of thrips adults on the remaining fields under production. This can often make chemical control very difficult, particularly in March, as thrips may continually re-infest fields following spray applications. Random Notes: Radiant (6-8 oz/ac) and Lannate (0.75 lb) + pyrethroid (high rate) are providing good residual efficacy of thrips in our trials at the Yuma Ag Center. Remember, the key to good thrips control is to prevent immature populations from becoming established. For more information on the identification, biology, ecology and management of thrips on desert produce please read: "Insect Management on Desert Produce Crops: Western Flower Thrips" and "Thrips Management in Desert Leafy Vegetables".
Alfalfa is a major crop grown in Yuma area as a cover crop before vegetable season. Generally expected to be a hardy and easy crop to grow, alfalfa production still can be affected by many pests and diseases. Root and crown rot in alfalfa is a common problem. The pathogen Phytopthora megasperma thrives on standing water, overwatered, and/or in soil that lacks good drainage.
The above ground symptoms typically consists of plant wilting, usually occurring quite suddenly, often followed by plant death. The roots turn tan-to-brown, sometimes turning black. Sometimes orange-to-reddish streaks can be observed in rotted roots. Occasionally, the symptoms can be seen in crowns too. Infected plants succumb to death, or grow at reduced rate thus being susceptible to other pests and diseases.
The causal organism survives in soil as mycelia in infected plant tissue or as thick-walled oospores. It also produces thin-walled sporangia that release motile zoospores in the presence of free water. So it is important to grow alfalfa in well drained soils. Adjust irrigation water in a way that there is no standing water for too long. If possible, install tailwater ditch to remove excess water. Plant resistant cultivars like Agate, Apollo if the problem persists in the field. Till the soil deeply to reduce wet soil pockets, level the land before planting.
If diagnosis is confusing, bring the samples to the Yuma Plant Health Clinic for proper diagnosis. Make sure to bring healthy looking plants, sick plant, plants in between etc. Remember, there is no such thing as too much plant tissue when it comes to submitting samples to the clinic!
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.
Results of pheromone and sticky trap catches can be viewed here
Corn earworm: Moth activity remains seasonably low, but continued to increase in the North Yuma Valley.Numbers are below average compared with previous seasons.
Beet armyworm: Moth activity decreased in the past 2 weeks and are about average compared with previous seasons.
Cabbage looper: Similarly, cabbage looper moth activity decreased and is below average compared with previous seasons.
Whitefly: Adult movement is relatively absent, but we picked up a few adults in the Yuma Valley; typical for this time of the season.
Thrips:Thrips activity up considerably consistent with the end of the produce season. About average compared with previous seasons for late March.
Aphids: Area-wide aphid flights increased slightly in the last 2 weeks. Most aphids found on traps were green peach aphid. About average movement for late March compared with previous seasons.
Leafminers: Adult activity picked up considerably in Yuma and Dome Valleys. Numbers are below average for this time of year.
Diamondback moth: Diamondback moth (DBM) counts in pheromone traps trended slightly upward over the past 2 weeks in most locations, and remained low in many locations with the exception of the Yuma Valley and Bard. 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.(See DBM Trap Network).