Effective control of western flower thrips (WFT) to prevent cosmetic scarring and contamination is important in spring lettuce crops, and now that INSV has been found infecting plants in Yuma lettuce, management becomes even more important.  For most of the growing season, WFT numbers have been below average based on sticky trap counts, field reports and population densities here at YAC. However, in the past week WFT populations have increased in most growing areas. This is not surprising as we typically observe “bioconcentration" of WFT during March and April on late lettuce as surrounding produce acreage declines. Each time a lettuce field is harvested, and disked, adult thrips disperse from these areas into the next available lettuce field.  As the number of lettuce acres becomes reduced near 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 of WFT very difficult, particularly in March, as adults can continually re-infest fields following spray applications.  Unfortunately, bioconcentration is likely to be even more intensified with the increase in late lettuce we have this season.  So, what management approaches can you take to manage thrips and hopefully reduce the potential incidence of INSV.  The first line of defense should be sanitation. Growers should disc under producecrops immediately following harvest. The longer a harvested crop remains above ground, the more insects that can build up and move to adjacent lettuce fields, especially with the dry, windy, and warm weather we typically have in March and April.  So far INSV incidence is light, and on average less than 1% infections has been reported in fields. However, the longer the season progresses and thrips begin to move, the threat of INSV spreading increases.
Controlling WFT with insecticides is the best approach to minimizing cosmetic feeding damage and may reduce spread of INSV within fields. For adults, which can easily be found on the leaf surface, your best choices of insecticide are methomyl (Lannate) or acephate at a high label rate.  They are the most efficacious products against adult WFT, but are also effective against larvae. Radiant is not as consistently efficacious against adults, but a 7 oz or higher rate will provide the best knockdown and residual control of WFT larvae. Torac, via contact activity, is relatively effective against WFT, and is most efficacious applied by ground on smaller plants when populations are lighter.  Remember, the key to preventing cosmetic damage by WFT is to maintain larval populations at low levels.  The cryptic or thigmotactic behavior of thrips often makes them difficult to find on lettuce plants. Research has shown that if you can see a few adults and larvae on the plant, it means that there are likely 8 to 10-fold more thrips actually on the plant (hiding near the base of the plant between midribs).  This behavior also means that spray coverage is important, particularly with contact insecticides like Lannate. There are other insecticides such as Minecto Pro, Exirel, Movento and Beleaf that can provide suppression (50% control or less) against WFT larvae, but don’t expect much activity against adults. For more information on the identification, biology, ecology, and management of thrips on desert produce please visit Western Flower Thrips Management in Desert Produce – 2023.

After more than 22 years of severe drought, watersheds across western North America are experiencing critical water levels that threaten agricultural and domestic supplies.  The Colorado River shortage is emblematic of these shortages.
 
These external factors are significantly impacting crop production systems at the field level in the desert Southwest.  Irrigation districts in central Arizona, particularly Pinal County, have lost access to Colorado River water allocations via the Central Arizona Project because of Tier 1 and Tier 2a reductions from the Drought Contingency Plan.  Reductions have also been imposed on Nevada and Mexico allocations.  Water levels in Lakes Powell and Mead are at less than 25% capacity and continue to decline due to insufficient action to arrest this worsening situation.
 
Since diversions to agriculture represent approximately 80% of the Colorado River water use, the basic arithmetic on the balancing of the river’s water budget will depend significantly on how agricultural diversions are ultimately managed in the process.  Further reductions of Colorado River water diversions to agricultural districts seems rather inevitable but many uncertainties remain with discussions and negotiations ongoing.
 
Water is the first and most limiting agronomic factor in crop production.  To deal with these challenging conditions from the water shortages at the field level agronomically, the crop and soil factors, there are some fundamentals that we can refer to for assistance.  
 
Dr. Jeremy Weiss, program manager for the University of Arizona AZMET system, has recently developed a valuable tool that provides actual crop evapotranspiration (ETc) estimates from several AZMET sites in the lower Colorado River Valley and for several key vegetable crops, including lettuce (iceberg and romaine), broccoli, cauliflower, cabbage, and spinach.  Accumulations of ETc values over the previous week and a range of dates are shown based on a date of planting, which is selected by the user.  In this model, the Kc values presented in FAO-56 are used for appropriate stages with each crop.  The reference evapotranspiration (ETo) measurements are taken directly from each AZMET site listed.
 
To access this crop-water estimate tool please refer to the following link:
 
https://viz.datascience.arizona.edu/azmet/azmet-crop-water-use-estimates/
 
For example, using lettuce (either iceberg or romaine) with a 1 November planting date through 11 December, we can see from this model the reference evaporation, listed as “water use” (ETo) and the cumulative crop evapotranspiration (ETc, or total crop-water use) of lettuce since the selected planting date.  
 
Since planting on 1 November through 11 December, a total crop water use, or the ETc, has been 3.57 inches in the Yuma Valley.  Last week, from 5-11 December, ETc has been 0.56 inches in the Yuma Valley.
 
Similar estimates are provided by this model for several AZMET weather stations in the Yuma production area: Roll, Yuma North Gila, Yuma South, and the Yuma Valley.  The Yuma Valley station is at the University of Arizona Yuma Agricultural Center and the Yuma South site is near Somerton, Arizona.  A map of AZMET sites can be found in the following link:
 
https://ag.arizona.edu/azmet/mapsites.htm
 
When we include the leaching requirement for the crop, an overall estimate of field level irrigation efficiency can be made.  We can estimate the leaching requirement as follows assuming an electrical conductivity for Colorado River water of 1.1 dS/m and the lettuce crop salinity tolerance of 1.3 dS/m.
 
 Leaching Requirement (LR) = ECw/((5 X ECe)-ECw)
 
LR = 1.1 dS/m / (5 X 1.3) – 1.1 = 1.1/5.4 = 0.20 = 20% leaching requirement
 
LR = 9.8 X 0.2 = 1.96 ~ 2 inches
 
Thus, total crop water demand in this case = 10 + 2 = 12 inches total
 
Contrasting this estimate of total crop-water use with the amount of irrigation water that was applied to the field can provide an estimate of the agronomic efficiency for a given field of lettuce.  We can do the same thing using this tool for other primary leafy green vegetable crops.
 
We are working in times of decreasing water availability and increasing scrutiny in agriculture.  It serves us well to measure and understand the relationship between crop demand (consumptive use plus leaching requirements) versus the irrigation water that is applied to a given field. 

DISEASE: Center Rot of Onion
PATHOGEN: Pantoea ananatis, Pantoea agglomerans, Pantoea alli and Pantoea stewartii subsp. indologenes
HOSTS: Onion (Allium cepa L.), garlic (Allium sativum L.), shallots (Allium cepa var. aggregatum L.), leeks (Allium ampeloprasum L.), chives (Allium schoenoprasum L.).

Symptoms and signs
Center rot of onion has not been a major problem in the desert southwest but when the environment is favorable, the disease can cause up to 90% loss. Foliar symptoms (symptoms on leaves) may start with water-soaked lesions spanning the length of the leaf blade, which gradually become blighted resulting in desiccation and collapse of the tissue. Experiments have shown that the bacteria can move from leaves to the bulbs, thus protecting foliage is important to manage the disease.  
 
The  bacteria can overseason to infect onions in a number of different ways.  Like many bacterial pathogens, P. ananatis can be seed-borne with infested seed serving as a survival mechanism as well as a means of dissemination. It has been demonstrated that P. ananatis can be both naturally seed-borne and seed-transmitted in onion. The significance of the bacterium's ability to colonize seed is uncertain, as most onion seed production sites are located in arid climates but extremely important to understand to manage the disease. 
Although P. ananatis can be seedborne, the proposed primary mode of transmission is by two insect vectors. Two species of thrips, tobacco thrips (Frankliniella fusca (Hinds)) and onion thrips (Thrips tabaci), have the ability to transiently acquire and transmit P. ananatis and P. agglomerans . The bacterium can persist in a non-circulative manner in the gut of thrips for 128 h, allowing the vector to infect plants over an extended period of time. 
 
P. ananatis can survive epiphytically and endophytically on a wide range of hosts. These alternative hosts can serve as a source of inoculum in fields where susceptible crops are grown. In Georgia alone, 25 weed species, including carpetweed (Mollugo verticillata), common ragweed (Ambrosia artemisiifolia), crabgrass (Digitaria sanguinalis), common cocklebur (Xanthium pensylvanicum), curly dock (Rumex crispus), Florida pusley (Richardia scabra), sicklepod (Cassia obtusifolia), stinkweed (Thlaspi arvense), Texas panicum (Panicum texanum), vaseygrass (Paspalum urvillei), wild radish (Brassica spp.), yellow nutsedge (Cyperus esculentus) and other multiple crop plants were found to harbor P. ananatis populations asymptomatically.

Pic Credit: Colton Tew


Center rot-onion pathosystem: Multiple sources of inoculum (seed, weed and
thrips) contribute to center rot epidemic in field; A: foliar symptom resulting in
reduced yield (pre-harvest) and B: bulb rot in storage resulting in post-harvest
losses

Onion cultivars resistant to Pantoea sp. are not commercially available. Use of certified onion seeds is encouraged to avoid introduction of Pantoea sp. inoculum in the production field. Planting early maturing or mid-maturing onion varieties are often recommended for growers. Late maturing varieties provide a larger window for infection and a potential epidemic to occur, which are favored by thrips pressure, hot and humid conditions, and lack of effective bactericides. Overhead irrigation should be avoided as it promotes bacterial spread compared with sub-surface or drip-irrigation. Controlling thrips population can be an effective management strategy to reduce center rot incidence as these vectors play an important role in bacterial transmission. 
Center rot management in onion fields relies heavily on copper applications mixed with an ethylenebisdithiocarbamate fungicide (EBDC), such as mancozeb, which growers may apply weekly as a protectant.  In addition, researchers found P. ananatis strains to be copper-tolerant indicating overuse and potential risk of insensitivity to this chemistry. Repeated applications of copper sprays during susceptible growth stages can be effective only to a limited extent and does not offer a robust solution to the problem. Perhaps the inefficacy of these sprays could be due to thrips preference to colonize certain parts of the onion plant, e.g. the basal meristems (neck region). 
 
The implementation of successful weed management strategies are important in reducing P. ananatis inoculum in the field. By reducing weeds, growers can potentially reduce initial inoculum. 
 
 
https://www.apsnet.org/edcenter/disimpactmngmnt/topc/Pages/CenterRotOnion.aspx

LAST CALL FOR MELON POWDERY MILDEW TRIALS
 
If you are planning to include your treatments in the melon powdery mildew field efficacy trial, this is the last  week to send the protocol. Contact Bindu Poudel-Ward (bpoudel@arizona.edu) for details and collaboration.

Despite the commercialization of numerous automated weeding machines, most fields are still hand weeded. A reason for this is that automated weeding machines cannot remove weeds close to the crop plant or are not highly efficient yet at doing so. 

A technique that I’ve been curious about for some time now to minimize the number of weeds close to crop plants is the use of early (seedling stage of growth) close cultivation. Early close cultivation can be accomplished using a camera guided cultivator equipped with specially designed cultivating tools. Examples of the technology and technique were demoed at our 2016 and 2022 AgTech Field Days by K.U.L.T.-Kress, LLC¹, the manufacturer of the equipment. The systems basically comprised a camera vision system for tracking the crop row, a toolbar attached to a parallel linkage that facilitated side-to-side movement and small cultivator assemblies for each of the seedlines (Fig. 1). In the demos conducted in lettuce, cultivating tools were positioned such that the uncultivated band was only about 2” wide (Fig. 2). If the crop were thinned by hand hoe or chemically, only a small area (roughly  2” x 2”) around the keeper plant would remain that had not been cultivated or treated with an herbicide material (Fig. 3). Assuming that existing weeds were controlled, and no new weeds emerge, the number of weeds close to the crop plant would be minimal. I realize soil disturbance near the crop plant is not desired but cultivating 1” away from seedling lettuce should be comparable to, and not cause any more crop injury than thinning seedlings spaced 2” apart with a hand hoe. I’m curious what your thoughts are on this. I’d love to hear your feedback.

[1] Reference to a product or company is for specific information only and does not
endorse or recommend that product or company to the exclusion of others that
may be suitable.

Fig. 1. K.U.L.T.-Kress camera guided cultivator (a) equipped with cultivating tools
designed for close cultivation (b). (Photo credits: Mazin Saber, University of
Arizona)


Fig. 2. Close cultivation of seedling lettuce with a camera guided
cultivator. Close up view (a) and far view (b). (Photo credits: Mazin Saber,
University of Arizona)


Fig. 3. Close cultivation of seedling lettuce with camera guided
cultivator leaves a roughly 2” wide uncultivated band around the
seedline (a). Thinning by hand hoe or chemically would kill plants in the
seedline (red shaded area) and leave a small area (roughly 2” x 2”)
(indicated by white box) around the keeper plant that had not been
cultivated or treated with an herbicidal material (b). Assuming no new
weeds emerge, the number of weeds near the crop plant that are
difficult and time consuming to remove is minimal.


Fig. 4. Video of camera guided cultivator equipped with cultivating tools
designed for close cultivation operating in six-line seedling romaine
lettuce. Click here or on the image to see the video. (Video credit: Mazin
Saber, University of Arizona)

 

The subject of Prefar (bensulide) residue in the soil and waiting period before spinach was was brought to the IPM team in the past. Inadvertently some sections of fields with bensulide residue could be scheduled for planting spinach too soon. The rotational crop instructions on the label say: “All other crops should not be planted for 120 days and the soil must be tilled to a minimum of 4 inches prior to replanting”. The Section VI of the PCA Sudy Guide recommends 4 months recropping interval for Alfalfa, wheat and Cotton. 
The site Gowan.com explains specifically: “Do not use Prefar 4-E on spinach or Swiss Chard as severe phytotoxicity will occur”¹.
We also heard different opinions, some minimizing the persistence on this herbicide in fine textured soils. So, we tried it in our clay soil at the Yuma Ag Center with a 40% Clay, 38% Sand, and 22% Silt.
After a bensulide lettuce evaluation was completed, we reworked the beds and planted spinach. This was done 45 days after treated.
Here’s how the stand was affected:

Figure1. Effect of bensulide herbicide residue in spinach stand. Broadcast “DI” means delayed incorporated and “II” Immediately incorporated with sprinkler irrigation. 
 
Additionally, leaves, roots and plant lengths were evaluated showing vigor reduction in treated plots.


Figure 2. Bensulide effects to spinach stand 45 planted 45DAT.

1. https://www.gowanco.com/products/prefar-4-e

Results of pheromone and sticky trap catches can be viewed here.
 
Corn earworm:           
CEW moth counts peaked about 2 weeks ago, but declined areawide this week. About average for October.
 
Beet armyworm:         
Trap counts increased in most locations, but remain well below average compared to previous years.
 
Cabbage looper:          
Cabbage looper up in most traps, particularly active in Yuma Valley and Wellton. About average for this time of season.
 
Diamondback moth:      
DBM moths peaked about 2 weeks ago (Dome and Wellton); below average for this time of the year.
 
Whitefly:                      
Adult movement increased in the past 2 weeks, particularly in areas near melons but remains below average for this time of year. 
 
Thrips:                           
Thrips adults increased markedly in many locations (Tacna, Dome, Yuma Valley), and currently about average for October.
 
Aphids:                        
Aphid movement beginning; light numbers in North Yuma Valley and Tacna. Below average for October.
 
Leafminers:                  
Adult activity up in a few locations (Yuma Valley), below average for September.