Historically, our Areawide Pheromone and Sticky Trap monitoring for insects was terminated around the first of April as the produce season ended. Beginning 4 years ago however, we continued our Areawide Trapping Network throughout the summer to collect trapping data from all 15 areawide trap locations year-round. So why is this additional trapping data useful? For several reasons:
1) Understanding the activity of some of our key pests when produce is not grown during the summer may give us an indication of what to expect as the fall produce season begins. This may be particularly helpful for predicting moth flights and whitefly flights in August-September coinciding with early transplanting and direct seeded crops. Another example is keeping track of corn earworm which can unexpectedly show up near the beginning of fall harvests.
2) Trapping for pests during the summer has shown us that 2 of our more important produce pests are not caught in traps during the summer. We presume this is due to the absence of brassica crops and weeds for diamondback moth, and high daytime/nighttime temperatures lethal to aphids. The fact that trap catches resume in the fall supports our conclusion that these pests are absent in the summer, only to reenter the desert via winds and/or transplants in the fall. And finally,
3) It gives me something to do in the summer other than write reports and papers.
So, visit the Areawide Summer Trap Network if you’re curious what our key pests are up to.
As we wrap up lettuce season and move towards melon season, bacterial wilt of cucurbit is a disease we have to keep an eye on. Couple of years ago we had high incidence of this disease in fields.
Bacterial wilt is a common occurrence in commercial fields and residential gardens. This destructive disease can potentially result in complete crop loss even before the first harvest. Hosts Cucumber and muskmelon (cantaloupe) are highly susceptible; squash and pumpkin are less susceptible; watermelon is resistant.
Initially, individual leaves or groups of leaves turn dull green and wilt (Figure 1), followed by wilting of entire runners or whole plants. At first, plants may partially recover at night, but as disease progresses, wilt becomes permanent. Collapsed foliage and vines turn brown (necrotic), shrivel, and die (Figure 2). Wilt symptoms may be noticeable in as few as 4 days from infection on highly susceptible hosts but can take up to several weeks to become evident on crops that are less susceptible. Plant growth stage can also affect disease progress, which is more rapid on young, succulent plant tissues.
The diagnostic feature for this disease is the emission of a slimy, sticky ooze (exudate made of polysaccharides and bacterial cells) from cut stems. Field diagnosis can be confirmed using a simple “bacterial ooze test.” With a sharp knife, cut through a wilted (but not dead) vine; use a section near the crown (Figure 3A). Touch the cut ends together, and then slowly pull them apart. Fine thread-like strands of bacterial ooze will be drawn out (Figure 3B) when bacteria are present. This test works well for cucumber and muskmelon but is less reliable for squash or pumpkin. If this disease is present, a cloudy string or mass of bacterial ooze will flow into the water from the cut stem pieces (Figure 3C).
Bacterial wilt is caused by Erwinia tracheiphila; striped and spotted cucumber beetles (Fig 4 and 5) serve as vectors, carrying the bacterium from plant to plant during the growing season. The life cycles of the bacterial wilt organism and its vectors are closely associated, and bacterial wilt is directly correlated to striped and spotted cucumber beetle populations. These beetles hibernate through winter under leaf litter and in other protected sites; all the while, the bacterial wilt pathogen overwinters within the gut of the striped cucumber beetle. The beetles become active once temperatures remain above 55°F in spring. As soon as cucurbit seedlings begin to break through the ground, the beetles begin to feed on cotyledons and later feed on leaves, stems, and flowers. Striped cucumber beetle larvae also feed on root systems, causing damage that can result in wilt. The bacterial wilt organism is deposited through beetle mouthparts and the frass deposited onto/ into wounds created during beetle feeding. Once the bacterium invades a plant’s water conducting vessels (xylem), it spreads rapidly throughout the plant. The matrix of bacteria and ooze obstructs water movement in the xylem vessels, which causes wilt symptoms. Further spread of the pathogen occurs when beetles feed on diseased plants and then feed on nearby healthy plants. Close to harvest, a second generation of striped cucumber beetle may acquire the bacterium while feeding on infected plant tissues. Fall-planted cucurbits may be infected by this generation. These late-season adults will overwinter with the live bacterium in their gut and possibly transmit the pathogen to young plants the next spring. The bacterium cannot survive in infected plant debris from one season to the next.
Prevention of bacterial infections is dependent upon preventing cucumber beetle vectors from feeding on cucurbit plants. Early protection is critical for long-term disease management, which should begin as soon as seedlings emerge or when plants are transplanted into fields or gardens. Once it is evident that plants are infected, they should be removed from the site and destroyed. An early, aggressive management approach has been shown to reduce amounts of disease later in the season.
Start an insecticide program as soon as seedlings emerge or immediately after transplanting. This is critical to protecting very small plants from beetle feeding and, ultimately, from bacterial wilt. Bactericides are not recommended for management of bacterial wilt disease. Plastic and reflective mulches, crop rotation have shown promising effect against the insects.
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.
