Now that the weather has finally returned to normal for mid-October, the worm pressure we’ve been experiencing should begin to decline.  However, with many early-planted head lettuce crops beginning to rosette and folding-in to form heads, it would be wise to keep a keen eye out for corn earworm (CEW).  This fall could be important as pheromone trap catches spiked last week in several locations (see graph below) moths continue to be active.  Although pheromone trap counts don’t always correlate to field infestations, we have associated peak CEW moth trap counts with larval CEW activity in surrounding lettuce fields in the past.  Even with the cooler temps expected this next week, moth activity should be expected in fields and that means eggs being deposited in or on developing heads.

CEW can be very damaging in early fall head lettuce crops. On older plants beginning to form heads, larvae will migrate to the succulent terminal growth. If not controlled before the plant leaves fold in, they are protected from foliar sprays. By this time in plant growth, at-planting soil applications of Coragen or Verimark may not be effective enough to protect the heads. Once head formation begins, newly hatched larvae will usually bore into the head almost at once upon hatching.  Corn earworm is much more likely to bore into lettuce heads than other Lepidoptera larvae, rendering the heads unmarketable.  Larvae may enter the head from any point on the plants but can often be found burrowing in from the lower half of the head. 

If fields are not watched closely, infestations may not be noticed until the head is harvested. Once inside the head, it is virtually impossible to control the larvae with insecticides. Thus, pay careful attention for newly oviposited eggs (laid singly) on lettuce plants. If you are beginning to find eggs and suspect that CEW are active in the field when plants are beginning to head or cup over, you should treat as soon as possible.  Moreover, during mid-late October and early November it is probably a good idea to prophylactically apply a pyrethroid, methomyl or acephate in combination with larvicides (e.g., Radiant, Coragen, Proclaim) when heads begin to form.  The UA nominal threshold for CEW in head lettuce from the beginning of heading to harvest is1-2 larvae / 100 plants (1-2%).  Repeated insecticide treatments may be required to maintain low population levels if heavy pressure is sustained near harvest. Lab bioassays have shown that CEW larval mortality is most rapid when exposed to Lannate at 0.5 lbs (>90% mortality in 1 hour after exposure) and pyrethroids at high rates (>90% mortality in 3 hours), followed by Radiant,5 oz (>90% mortality in 12 hours).  By 24 hours, mortality was 100% for all the treatments including Coragen and Proclaim.  For more information on CEW management see Corn Earworm Management on Desert Produce.

Nutrient Mobility Concept
In a recent article published in this newsletter on 27 November 2024, Volume 15, No. 24, I presented an article addressing soil health and the Bray Nutrient Mobility Concept in relation to mobile nutrients (Silvertooth, 2024).  In this article, the concept of nutrient behavior in soil-plant systems focuses on the immobile plant nutrients.

Plant nutrient management is strongly dependent on nutrient mobility in the soil.  Nutrient mobility in the soil is different among the essential plant nutrients and nutrient management in the field needs to take this into account.

In 1954, Dr. Roger H. Bray at the University of Illinois proposed a nutrient mobility concept that has proven to be very important in the management of nutrients for optimum efficiency (agronomically, economically, and environmentally).  Bray essentially simplified all soil nutrient chemistry to the fact that some plant nutrients are mobile in the soil and some are not. (Bray, 1954; Raun, 2017; Warren et al., 2017, Havlin et al. 2014; Troeh and Thompson, 2005).

 

Mobile Nutrients and the Root System Sorption Zone
Mobile plant nutrients in the soil move with the soil water.  Thus, plants can extract mobile nutrients from a large volume of soil beyond the direct root system.  Accordingly, plants take up mobile nutrients from a “root system sorption zone” (Figure 1).  This gives plants the capacity to utilize most of the mobile nutrients in the root system sorption zone as those nutrients will move to the plant roots with the soil water as it is taken up by the plant (Silvertooth, 2024).

We consider the mobile plant nutrients to be nitrogen (N), sulfur (S), boron (B), and chlorine (Cl).  These mobile plant nutrients are taken up by the plant in the following forms: nitrate-nitrogen (NO₃^--N), sulfate-sulfur (SO₄^2- - S), boric acid (H₃BO₃) and borate ions (BO₃^3- - B), and chlorine is taken up as the chloride ion (Cl^-).

Figure 1.  The root system sorption zone and an illustration of the large volume of soil
from which plants extract mobile nutrients.

In a crop field where many plants are growing together, there are commonly root system sorption zones commonly overlap.  Therefore, the root system sorption zones for adjoining plants are competing for water and mobile nutrients, (Figure 2).  This is one of the main reasons that appropriate plant populations are important for optimum yield.

Figure 2. Competition among plants brought about by increasing yield goal.

Immobile Nutrients
Plant nutrients that are immobile in the soil include phosphorus(P), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), and molybdenum (Mo).  Immobile nutrients do not move as freely in the soil solution as the mobile nutrients do. These nutrients interact more directly with soil colloids and root surfaces. 

Immobile nutrients are absorbed by the plant from the soil and soil solution that is directly next to the root surface.  Plant roots must grow through the soil volume to come into direct contact with the immobile nutrients.

Thus, only a small volume of soil and soil solution that immediately adjacent to the root surface will be involved in providing immobile nutrients to the plant.  Figure 3 describes this soil volume and plant root interface as the root surface sorption zone.

Figure 3.  The root surface sorption zone and an illustration of the small volume of soil
from which plants extract immobile nutrients. 

In the case of immobile nutrients, the entire soil volume is not as important as the soil colloid surfaces and soil solution next to the root surface.  The concentration of immobile nutrients on the soil colloids immediately next to the root surface is a critical of the root system sorption zone.

Since only a thin layer of soil surrounding and in direct contact with the plant roots are involved in supplying immobile nutrients to the plant, there is little or no competition among plants for immobile nutrients.  Competition among plants only occurs at points where roots from adjacent plants come in direct contact with one another (Figure 4).

Figure 4.  Limited competition among plants for immobile nutrients.

Due to this manner of immobile nutrient behavior and interaction with plant roots, the supply or concentration of immobile nutrients such as phosphorus (P) or potassium (K), is not dependent on a yield goal.  In the case of immobile nutrients, the overall soil concentration of the immobile nutrients is most important, and these nutrients are not moving readily with soil water.  If the immobile nutrient supply in the soil is adequate for optimum yield of a crop, the healthy plant root system can explore new soil volume and extract the nutrient sufficiently, such as phosphorus (P) or potassium (K).

The nutrient mobility concept and these basic illustrations can help us understand the basis for some common observations and resultant crop management practices.  Fertilizers with immobile plant nutrients are more effective when they are incorporated into soil and particularly in soil zones where there is a high probability of plant roots encountering the immobile nutrients.

Banded applications of immobile nutrients are generally more effective than the same rates broadcast and incorporated into the soil.  In contrast, mobile nutrients like nitrogen (N) can be broadcast and moved into the root system sorption zone by water.

Soil tests for immobile nutrients do not normally change much from year to year and this is true irrespective of the crop yields from the previous season or fertilizer rate. This is because most of soil volume was not in direct contact with the plant roots.  Soil concentrations of immobile nutrients do not usually change rapidly but they can be slowly mined out of the soil by a series of crops without proper fertilization.

Continued or over-applications of immobile nutrient fertilizers, such as phosphorus (P), will cause a buildup of that nutrient in the soil.  This is because only a small fraction (commonly 15-20% for most crops) of the nutrient or fertilizer comes into direct contact with the plant roots. The remaining amount of fertilizer interacts with the soil.

Appropriate soil tests that are properly correlated and calibrated with crop-specific response categories are important in evaluating immobile plant nutrient status.  Immobile nutrient levels in the soil are commonly expressed in terms of percent sufficiency to produce a specific crop based on appropriate soil test results.

Our goal in plant nutrition management is to achieve the highest levels of efficiency (agronomically, economically, and environmentally) in the field as possible

References:
Bray, R.H.1954.  A Nutrient Mobility Concept of soil-plant relationships.  Soil Sci. 78(1), p. 9-22.

Havlin, J.L., Beaton, J.D., Tisdale, S.L. and Nelson, W.L. 2014.  Soil Fertility and Fertilizers; An Introduction to Nutrient Management.  6th Edition, Prentice Hall, Upper Saddle River, NJ.

Silvertooth, J.C.  2024. Soil Health - Bray’s Nutrient Mobility Concept and Mobile Plant Nutrients University of Arizona Vegetable IPM Newsletter, Volume 15, No. 24,

Raun, W.R. 2017.  In: Warren et al.  2017. Oklahoma Soil Fertility Handbook, Id:E-1039

Troeh, F.R. and Thompson, L.M. (2005) Soils and Soil Fertility. Sixth Edition, Blackwell, Ames, Iowa, 489.

Warren, J., H. Zhang, B. Arnall, J. Bushong, B. Raun, C. Penn, and J. Abit. 2017. Oklahoma Soil Fertility Handbook. Id: E-1039

Weil, R.R. and Brady, N.C. (2017) The Nature and Properties of Soils. 15th Edition, Pearson, New York.

In the past few weeks we have seen increase in cucurbit samples submitted to the plant disease diagnostic clinic infected with bacterial wilt. PCAs have also reported increase in number of cucumber beetle in the 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.
 
More information: http://plantpathology.ca.uky.edu/

With harvest season in full swing, I thought I would share with you an observation I’ve made over the years. I’ve noticed that gulls seem to congregate around fields being harvested. I am not sure what the attraction is, but my guess is that they are begging for food from harvest crews. Gulls are problematic in that they are known to be carriers of several human pathogens including E. coli O157, Salmonella and Campylobacter (Jay-Russell,2013). When gulls fly over fields, it is inevitable that some of their fecal matter will be deposited on unharvested crops, in irrigation canals and on harvest equipment (Figs 1-4). Obviously, when this happens, it is a food safety risk. Despite the significant effort and investment made in bird deterrent technologies, I am not aware of any that are effective and reasonably priced for gulls. The best advice I have for reducing the risk of food contamination from gulls is to be vigilant about instructing workers not to feed the birds and locate food trucks away from crops to be harvested.

References
Jay-Russell, M.T. (2013). What is the risk from wild animalsin food-borne pathogen contamination of plants?. CABI Reviews 4(8),1-16.https://doi: 10.1079/PAVSNNR20138040

Fig. 1. Bird fecal matter on romaine lettuce.


Fig. 2. Gull flying over romaine lettuce being harvested.


Fig. 3. Gulls flying over irrigation canal near lettuce field
being harvested.


Fig. 4. Bird fecal matter on lettuce harvesting equipment.

Thank You

In 2010 we started publishing the Arizona Vegetable IPM Updates to a small number of friends. It was embraced by the community because of contributions from Mike Matheron, John Palumbo and Barry Tickes in the Plant Pathology, Entomology and Weed Science areas. Mike, John and Barry are the original IPM dudes. 

I was honored to be part of the team editing and sending the Newsletter for almost
16 years. 


This is my last update, and I would like to thank the UA team and the agricultural community for the opportunity to work with you. 

Western Flower thrips is among the most economically important insect pests that infests desert lettuce. Bean thrips started to become a major pest of fall lettuce in the desert over the last decade. Our objective is to determine alternative organic insecticides that can be used as part of an IPM program targeting these pests in leafy vegetable crops.

This fall, at the YAC experimental farm, we evaluated 10 organic insecticides frothier efficacy in suppressing western flower thrips and bean thrips in organic head lettuce. M-Pede, Pyganic, and Aza-Direct were evaluated with Oroboost included in one treatment and Orbit DL included in the other treatment. Thus, we evaluated these aforementioned insecticides in two different treatment entries. The purpose was to understand the performance of these insecticides with Orbit DL or Oroboost as adjuvant. The remaining insecticide treatments had only Oroboost as adjuvant. Both Oroboost and Orbit DL are OMRI approved organic adjuvants.

We expected that several of these bioinsecticides would exhibit some measurable level of thrips suppression. However, our data showed that only Entrust exhibited measurable reduction in immature thrips, western flower thrips, and bean thrips numbers (Fig. 1-3). Gargoil and Pyganic with the Orbit as adjuvant exhibited a slight reduction in western flower thrips adults (Fig. 2). We will continue to evaluate these organic insecticides against the thrips to gather more research-based evidence which will allow us to draw more accurate conclusions and make relevant recommendations.

Figure 1. Mean thrips nymph/plant as affected by organic insecticide application.
DAT=Day After Treatment.


Figure 2. Mean western flower thrips adult/plant as affected by organic insecticide
application. DAT=Day After Treatment.


Figure 3. Mean bean thrips adult/plant as affected by organic insecticide application.
DAT=Day After Treatment.

 Area wide Insect Trapping Network   VegIPM Update, Vol. 15, No. 2, Jan 24, 2024
 
Results of pheromone and sticky trap catches can be viewed here.
 
Corn earworm:              
CEW moth counts are down areawide and average for this time of year. 
 
Beet armyworm:          
Trap counts decreased in most locations; about average compared to previous years.
 
Cabbage looper:         
Cabbage looper counts increased in most traps and about average for this time of season.
 
Diamondback moth:      
DBM moths continue to increase in all areas. Below average for this time of the year.
 
Whitefly:                      
Adult movement negligible, typical for early January. 
 
Thrips: 
Currently, numbers are about average compared with previous years.
 
Aphids:                        
Aphid movement increased significantly in the past two weeks, particularly in North Yuma and Gila Valleys. Well above average for early January.
 
Leafminers:                   
Adults decreased in most locations, but above average for January.