Fall melon production is underway and transplanting/direct seeding of produce crops is not far behind. So, PCAs will soon be faced with several important insect management decisions.  As crops begin to emerge, you can expect to encounter several insect species that can potentially cause serious economic losses to crop stands.  These seedling pests include flea beetles, crickets, grasshoppers, darkling beetles, earwigs, and saltmarsh caterpillars (‘woolly worms’). These insects all have chewing mouthparts, and most are capable of consuming large amounts of leaf tissue in a short period of time. Seedling crops at the cotyledon stage are most susceptible; these pests can devour entire cotyledons or outright kill small seedlings.  If left unprotected, transplants and larger seedling plants can sustain significant feeding damage on the terminal growing points or newly emerged leaves.  Not only can this feeding stunt plant growth but can result in lack of stand uniformity and ultimately, lack of uniform maturity at harvest. Host crop sources for flea beetle, crickets and "woolly worm" infestations include several summer crops (e.g., Sudan grass, cotton, and alfalfa), volunteer melons, and weeds (e.g., purslane, pigweed). I have been seeing a considerable number of pale stripped flea beetles on alfalfa recently. Fortunately, growers have been vigilant about field sanitation, and weeds are few and far between throughout the valleys. Crickets have also been especially abundant this summer, so keep a sharp eye for them, particularly under sprinkler pipe.    Salt marsh caterpillars have not been reported to date, but are known to disperse from alfalfa and cotton, particularly Pima cotton (non-Bt).  Experience suggests that melon fields planted next to these crops/weedy areas are at a high risk from these seedling pests, particularly flea beetles. As summer crops are harvested or terminated during the next several weeks, these seedling pests typically move to the next available host crop; lettuce, cole crops and melons.  Fortunately, there are many registered insecticide alternatives available that can be applied via sprinkler chemigation (i.e., pyrethroids) or foliar sprays (i.e., pyrethroids, methomyl, neonicotinoids) that can cost-effectively minimize their abundance and damage to susceptible produce and melon seedlings. Additionally, insecticide seed treatments are available for lettuce and broccoli that will protect stands from flea beetles and bagrada bug (i.e., NipsIt).  For more information on insect pests of leafy vegetables and melons at stand establishment please see Insect Management on Desert Produce and Melons: Pests at Stand Establishment.

Nutrient Mobility Concept
In two recent articles published in this newsletter on 27 November 2024, Volume 15, No. 24 and 25, I presented the Bray Nutrient Mobility Concept (Bray, 1954) in relation to mobile and immobile nutrients (Silvertooth,2024a and Silvertooth, 2024b).  This article provides a summary of the nutrient mobility concept and the implications of mobile and immobile nutrient behavior in soil-plant systems and plant nutrition management. 

In 1954, Dr. Roger H. Bray at the University of Illinois proposed a nutrient mobility concept that has proven to be very important inthe 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 soil water as it is taken up by the plant (Silvertooth, 2024a).

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 form: 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.

Root system sorption zones of adjacent plants commonly overlap, and plants compete for water and mobile nutrients (Figure 2) in the soil and for light at the canopy surface.  This is one of the reasons 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.

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. 

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.

Plant Nutrient Management Implications
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 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.  On the other hand, continued or over-applications of immobile nutrient fertilizers, such as phosphorus (P), will cause a buildup of that nutrient in the soil since only a small fraction (commonly 15-20% for most crops) of the nutrient or fertilizer comes into direct contact with the plant roots. 

Appropriate soil tests that are properly correlated and calibrated with crop-specific response categories are important in evaluating immobile plant nutrient status.  

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.  6thEdition, Prentice Hall, Upper Saddle River, NJ.

Silvertooth,J.C.  2024a. Soil Health - Bray’sNutrient Mobility Concept and Mobile Plant Nutrients University of ArizonaVegetable IPM Newsletter, Volume 15, No. 24.

Silvertooth, J.C.  2024b. Soil Health - Bray’s Nutrient Mobility Concept and Immobile Plant Nutrients University of Arizona Vegetable IPM Newsletter, Volume 15, No. 25,

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.

Bacterial fruit blotch (BFB) of melon is caused by the bacterium Acidovorax avenae subsp. Citrulli. The bacteria produces large olive green to brown water-soaked lesions on fruit, making them unmarketable. Symptoms of BFB on seedlings begin with water-soaked areas on the lower surface of the cotyledons and inconspicuous lesions on leaves. BFB lesions will become necrotic often with yellow halos. Lesions are frequently delimited by veins. Infected seedlings collapse and die. Greenhouse conditions are usually favorable for dispersal and establishment of pathogen. Thus, good greenhouse practices and sanitation is extremely important. Clean transplant trays must be used (disinfect trays if they will be reused) and new soil. Destroy any volunteer seedlings and keep the area in and around the greenhouse weed free. Avoid overhead watering if at all possible, or water in the middle of the day so that the plants dry thoroughly before evening. The bacterium can spread on mist and aerosols. Relative humidity should be kept low through proper watering and good air circulation in the greenhouse. Separate different seedlots, to reduce lot-to-lot spread. Monitor these isolated seedlings daily and destroy trays where symptoms develop. The remaining trays should be sprayed with a labeled bactericide and the applications continued until the plants are transplanted to the field. The pathogen can be seedborne, so growers should only use seed that has been tested for the presence of the pathogen by a reputable testing facility. Management of BFB includes a combination of preventing the introduction of the pathogen, sanitation to eliminate any inoculum present, and the use of bactericides if the disease appears. There are no commercially available watermelon cultivars that are resistant to bacterial fruit blotch, but there is some variation in susceptibility among cultivars. 

PBS News recently did a story on the importance of Yuma’s winter vegetable production and the pressures the Western US is facing due to water scarcity. The story features insights from local growers Matt McGuire, JV Smith Companies and Jon Dinsmore, Dinsmore Farms, as well as Arizona Department of Agriculture Director, Paul Brierley. The report is well done, covering the basics of Yuma agriculture and the water shortage issues the industry is facing. It’s a quality piece, worth a listen.

Check it out by clicking here or on the image below.

Fig. 1. PBS NewsHour: “Arizona Farmers Forced to Adapt as Main Water Source Dries Up” story. (Photo Credits: PBS News)

 

In the August 21, 2024 update we mentioned our plans to do a demonstrative trial at the Yuma agricultural center. This experiment was established on 10/2 and planted on 10/14. Sprinklers started on10/15. A stand evaluation was done on 10/28 and Phytotoxicity and Goosefoot and Lambs quarter weed control evaluated yesterday. The treatments suggested by some of the people in the industry, where:

Treflan (trifluralin) Applied at 1.5pt/A pre-emergent on the flat ground incorporated mechanically by disking approximately at 4” depth, then make the beds and plant the crop to germinate with sprinkler irrigation. The same method was utilized for Prowl herbicide at the rate of 1 pint product per acre. Also Prowl non-incorporated was included, Devrinol2XL at 1 pt/A, Prefar at 6qt/A as well as a high rate of Goal (oxifuorfen)preemergence to evaluate and document phytotoxicity levels.

As mentioned, we have collected weed control data, which we plan to include in future updates.

Would you like to see how this treatments look? Some of the observations, even though expected are very interesting, such as the fact that 16 floz of Goal Tender preemergence didn’t allow goosefoot germination but similarly broccoli didn’t emerge.

Come to the NW corner of the Ag Center (you can’t miss it) and look at the plots and see the difference between Prowl mechanically incorporated and non-incorporated, compare it with Prefar and Devrinol. We put some very clear signs as you can see in the picture above. The experimental plots are 14x60ft so they are large enough to illustrate what would happen in a farm.

We thank you for your suggestions ...please come and let us know what you think!

As the weather gets cooler, aphids are migrating back to the desert to harm our crops. It is important to revisit some factors that may favor aphid infestations in your crops, specifically lettuce, and take appropriate action against them.  

Nitrogen availability is one of the most important factors in the development of insect pest populations. Excessive application of nitrogen fertilizer to crops will likely increase the feeding preference and consumption rate of insect pests such as aphids, resulting in greater survival, growth, and reproduction. In some situations, high nitrogen levels in plant tissue can decrease resistance and increase susceptibility to aphids’ attacks.

Water management is also very important to consider. Excess water around plant roots can increase nitrogen uptake, which may favor an increase in the aphid population. Thus, supplying proper amounts of nitrogen and water to your crops can tremendously help with aphid management. See this link for more information on this topic.

 Area wide Insect Trapping Network   VegIPM Update, Vol. 14, No.7, Apr 5, 2023

Results of pheromone and sticky trap catches can be viewed here.
 
Corn earworm:            
CEW moth counts increased sharlply in most trap locations this week; above average for this time of year. 
 
Beet armyworm:         
Trap counts increased sharply over the past week; above 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 remained low in most areas and well below average for this time of the year.
 
Whitefly:                      
Adult movement was negligible and typical for this time of year. 
 
Thrips:                          
Thrips adult counts increased slightly over the past 2 weeks. Currently, well below average compared with previous years.
 
Aphids:                        
Aphid movement decreased at all locations, and below average activity for late-March.
 
Leafminers:                  
Adult activity up in some locations, and below average for late-March.