Spring melon crops are rapidly growing, and insect pest populations are not far behind. Cabbage loopers and leafminers are starting to show up, and PCAs should start ramping up their insect monitoring. Whitely populations, albeit light for this time of the year, are slowly beginning to show on spring melons of all sizes. Adults can easily be found on recently planted melons located at the Yuma Ag Center, and reports from local PCAs suggest that adult populations are beginning to show up on older plantings. As temperatures increase and crops/weeds mature, avoidance of excessive feeding from whitefly nymphs should be the primary concern on all melon types. Although CYSDV occurs in later spring melons, it is rarely yield limiting. But honeydew and sooty mold contamination on cantaloupes, mixed melons and watermelons can significantly reduce quality and marketability if whiteflies are not adequately controlled.  Our research has shown that to prevent fruit yield and quality losses on spring melons, a foliar insecticide treatment should be applied on threshold; that is, when leaves have greater than 2 adult whiteflies per leaf, when averaged across an entire melon field. At this level of adult abundance, immature populations are just beginning to colonize. Timing sprays based on this adult threshold has been shown to significantly reduce yield / quality losses during spring harvests.  This threshold applies for the use of recommended IGRs (Courier, Knack, Cormoran, and Oberon), foliar applied neonicotinoids (Assail, Venom, Scorpion), neonicotinoid-like compounds (Sivanto prime and Transform), diamides (Exirel and Minecto Pro), and the new feeding disruptors (PQZ and Sefina).  For more information on whitefly management and available insecticides, go to these documents on IPM and Whitefly Management and Whitefly Control Options-Spring 2022. Also, be aware of honey bees and other pollinators in or around melon fields. If bees are present, be sure to carefully read labels and determine bee safety of a product before making an application in a melon field.  If applications are necessary during bloom, only apply a product that is considered bee safe (e.g., PQZ, Sefina, Sivanto, Assail). We also recommend that insecticides only be applied when honeybees are not actively working in the field (e.g., 10:00 pm – 3:00 am). 

In arid and semi-arid regions, water is our first limiting factor in a crop production system, followed closely by bio-available nitrogen (N).  Thus, our management of water and N are critically important to produce a healthy crop with good yields and quality.
 
Water and nutrient demands coincide with the fruiting cycle and efficient management of irrigation water and plant nutrients is enhanced by tracking crop development in the field.  The use of heat units (HUs) with 86/55 ^oF upper and lower thresholds can be applied to warm season crops in the desert Southwest in relation to the thermal environmental impacts on the development of all crop systems (Brown, 1989), including chiles, (Figures 1 and 2).
 
Crop Phenology Relationship to Water and Nitrogen Demand
 
Phenological guidelines have been developed for many crops, including New Mexico type chiles (Soto-Ortiz and Silvertooth, 2007 and Silvertooth, et al, 2010; Figure 1).  This phenological guideline can be used to identify or predict important stages of crop development that impact physiological requirements.  For example, a phenological guideline can help identify stages of growth in relation to crop water use (consumptive use) and nutrient uptake patterns (Figure 3).
 
This information allows growers to improve the timing of water and N inputs to improve production efficiency.  For some crops or production situations HU based phenological guidelines can be used to project critical dates such as harvest or crop termination.  Many other applications related to crop management (e.g., pest management) can be derived from a better understanding of crop growth and development patterns.

Figure 1.  Typical relationship between the rate of plant growth and development
and temperature.  Growth and development ceases when temperatures decline
below the lower temperature threshold (A) or increase above the upper
temperature threshold (C).  Growth and development increases rapidly when
temperatures fall between the lower and upper temperature thresholds (B).


Figure 2.  Basic phenological guideline for irrigated New Mexico-type chiles.

References
Brown, P. W.  1989.  Heat units.  Bull. 8915, Univ. of Arizona Cooperative Extension, College of Ag., Tucson, AZ.
Silvertooth, J.C., P.W. Brown, and S. Walker.  2010. Crop Growth and Development for Irrigated Chile (Capsicum annuum).  University of Arizona Cooperative Extension Bulletin No. AZ 1529
Soto-Ortiz, R. and J.C. Silvertooth.  2007. A Crop Phenology Model for Irrigated New Mexico Chile (Capsicum annuum L.) The 2007 Vegetable Report. Jan 08:104-122. 

Accepted in “Plant Disease”  April 2023
Neeraja Singh^†, Jeff Klingenberg, and Bindu Poudel-Ward
Stevia (Stevia rebaudiana Bertoni) is an important medicinal crop grown worldwide. Leaves of stevia contain a non-caloric sweetener, stevioside, which is used as a substitute to artificial sweeteners. In August 2022, symptoms of chlorosis, wilting, and root rot were observed in about 30 % of stevia plants growing at the Agricultural Station at Yuma Agricultural Center, Yuma, AZ (32.7125° N, 114.7067° W). Infected plants initially showed chlorosis and wilting, and the plants eventually died with the foliage remaining intact to the plant. Cross sections of the crown tissue of affected stevia plants showed necrotic tissue and a dark brown discoloration in the areas of the vascular and cortical tissues. Dark brown microsclerotia were observed on stem bases and necrotic roots of the infected plants. Symptomatic plants were sampled to isolate the pathogen. Root and crown tissues (0.5 to 1 cm) were surface-disinfested with 1 % sodium hypochlorite (NaOCl) for 2 min, rinsed three times with sterile water and were plated onto potato dextrose agar (PDA). Fungal isolate (Yuma) displayed rapid mycelial growth on PDA at 28°C with a 12-h photoperiod regime. The mycelia were initially hyaline and turned from gray to black after 7 days. Masses of dark spherical to oblong microsclerotia were observed after 3 days on PDA, measuring an average of 75 µm width x 114 µm length (n=30). For molecular identification, genomic DNA was isolated from sclerotia using the DNeasy Plant kit (Qiagen). The internal transcribed spacer (ITS), translation elongation factor- 1α (TEF- 1α), calmodulin (CAL), and β-tubulin (β-TUB) regions were amplified using the primer sets, ITS1/ITS4 (White et al. 1990), EF1-728F/EF1-986R (Carbone and Kohn, 1999), MpCalF/MpCalR (Santos et al. 2020), and T1/T22 (O’Donnell and Cigelink, 1997), respectively. A BLAST search of sequences revealed 98.7 to 100-% identity to Macrophomina phaseolina sequences (MK757624.1, KT261797.1, MK447823.1, MK447918.1). Both morphological and molecular characteristics confirmed the fungus as M. phaseolina(Holliday and Punithaligam, 1970). Sequences were submitted in the GenBank under accession numbers OP599770.1 (ITS), OP690156.1 (TEF-1 α), OP612814.1 (CAL), and OP690157 (β-TUB). Pathogenicity assay was performed on 9-week-old stevia plants, var. SW2267, grown in 4-inch planters in the greenhouse. The inoculum was made from a 14-day-old culture of M. phaseolina grown in conical flasks (250 ml) in potato dextrose broth (pH 5.6) at 28°C with 12 h of daylight. Mycelial mats of the fungus were blended in 250 ml of sterile distilled water, filtered through four layers of cheesecloth, and then calibrated to 10⁵ microsclerotia/ml using a hemocytometer. Twenty-healthy-plants were inoculated by soil drenching 50 ml of sterile water containing 10⁵ microsclerotia/ml per pot. Five non-inoculated plants were kept as control. Plants were maintained in the greenhouse at 28 ± 3°C with 12 h photoperiod. After 6 weeks, necrosis at the base of petioles and chlorosis of the leaves, followed by wilting were noticed on inoculated plants, whereas the control plants remained healthy. The fungus was reisolated and identified as M. phaseolina based on the morphology and sequencing analysis. Although M. phaseolina has been reported earlier on stevia in North Carolina (Koehler and Shew, 2018), this is a first report from Arizona. M. phaseolina is known to be favored by high soil temperatures (Zveibil et al. 2011), thus represents a potential threat to stevia production in Arizona in coming years.

Supplementary Figure. Signs of naturally occurring charcoal rot infection on Stevia
rebaudiana caused by Macrophomina phaseolina (A, B); root necrosis (C); discoloration
of root tissue in the cross-section (D); microsclerotia underneath the bark tissue (E);
microsclerotia from a pure culture (F).  

References:
Carbone, I., and Kohn, L. M. 1999. Mycologia 91:553. https://doi.org/10.1080 /00275514.1999.120-61051

Holliday, P., and Punithaligam, E. 1970. Macrophomina phaseolina. Commonwealth Mycological Institute Descriptions
of Pathogenic Fungi and Bacteria, No. 275. Commonwealth Mycological Institute. Kew, UK.

Koehler, A. M., and Shew, H. D. 2018. Plant Dis. 102:241. https://doi.org/10.1094/PDIS-05-17-0693-PDN

O’Donnell, K., and Cigelink, E. 1997. Mol. Phylogenet. Evol. 7:103. https://doi.org 10.1006/mpev.1996.0376

Santos, K. M., et al. 2020. Eur. J. Plant Pathol. 156:1213. https://doi.org/10.1007/s10658-020-01952-8

White, T. J., et al. 1990. Page 315 in: PCR Protocols: A Guide to Methods and Applications. M.  A. Innis et al., eds. Academic Press Inc., New York.

Zveibil, A., et al. 2011. Plant Dis. 96:265. https://doi.org/10.1094/PDIS-04-11-0299

At the UC Cooperative Extension 2024 Automated Technology Field Day in Salinas, CA a couple of weeks ago, 12 of the latest automated and robotic technologies were demonstrated in the field. Most were designed for weed control or thinning vegetable crops. Several of the technologies shown are relatively “new” for the 2024 season. These included laser weeders that also are capable of thinning lettuce (Fig. 1 & 2), a smart cultivator/side-dresser that cultivates between individual crop plants and simultaneously applies fertilizer at a variable rate depending on plant size (Fig. 3), a spot sprayer that utilizes superheated vegetable oil to kill weeds (Fig. 4), and a self-propelled machine that disinfests soil prior to planting using steam. Although the test runs were short, I was impressed with the possibilities for these machines. Company representatives said they will be in Arizona this upcoming season and are interested in meeting with growers. Company contact information can be found at their respective websites, or feel free to contact me if you would like additional information.

Fig. 1. Carbon Robotics’¹ (Seattle, WA) LaserWeeder^TM. The unit utilizes a vision
system to detect crop plants and weeds. A laser is used to kill unwanted plants.
The machine can be used for weeding or thinning lettuce crops (bottom).


Fig. 2. L&A¹ (Chico, WA) autonomous laser weeding/thinning robot. The unit is
equipped with a vision system to detect crop plants and weeds, and a laser to kill
unwanted plants. The machine can be used for crop thinning or weeding (weeded
carrot crop, bottom).


Fig. 3. Stout Industrial Technology’s¹ (Salinas, CA) smart cultivator/side
dresser. The unit is equipped with a vision system for detecting crop plants, and
blades that move in and out of the crop row to remove in-row weeds. Liquid
fertilizer (shown colored with blue dye) is applied at a variable rate depending on
plant size.


Fig. 4. Tensorfield Agriculture¹ (Union City, CA) precision spot spray weeding
machine. The unit has an 80” wide spray boom equipped with 232 individually
controllable spray nozzles to spot spray weeds with superheated vegetable oil
(bottom left, bottom right). A vision system is used to detect weeds and spot
spray resolution is ¼”.


Fig. 5. UA/UC Davis self-propelled band-steam applicator. Device injects steam
into the soil to kill weed seed and soilborne pathogens prior to planting.

[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.

Question to IPM team:
I applied Raptor and Pursuit to an alfalfa field. Can you provide some information on the carryover and phytotoxicity to onions?
IPM Team:      We haven’t done plant back evaluations with these products. The soil persistence that is mentioned in the AZ PCA study guide is 2-4 months for the Raptor and 3-12 months for Pursuit. The label specifies longer replanting intervals for various crops and suggests that “before planting any crop not listed elsewhere in Rotational Crop Restrictions, a successful field bioassay must be completed. The field bioassay consists of a test strip of the intended rotational crop planted across the previously treated field and grown to maturity”.
An interesting journal article “Injury to Vegetable Crops from Herbicides Applied in Previous Years”, mentions In 1995, residual imazethapyr delayed tomato maturity but did not reduce tomato yield. Other vegetable crops were not injured by herbicide residues. Then in 1997 “imazethapyr carryover injured cabbage, onion, and tomato plants and reduced tomato yield most injury occurred at 2X and 4X rates¹.
The “Carrington Research Center in NDSU did an herbicide carryover study included here which presents the “response of onions to herbicides applied to soybean the previous year²” including Raptor and Pursuit.  Also, in this study (table below) onion injury was significant in the 2X and 4X rates.

Another “Evaluation of Pre and Post Emergence Herbicides on Yield Contributing and Quality Characters in Onion” found that “Imazethapyr @ 100 g a.i / ha as post emergence application (20 DAT) coupled with preemergence herbicides produced the lower (yield) values than weedy check as Imazethapyr found to be toxic to the Onion^3”.
The field study “Efficacy of imidazolinone herbicides applied to imidazolinone resistant maize and their carryover effect on rotational crops” showed sensitivity of rotational crops, from high to low, was the following: Beta vulgaris>Capsicum annum>Lycopersicum esculentum>Cucumis melo>Hordeum vulgare>Medicago sativa>Lolium multiflorum>Avena sativa>Pisum sativum>Allium cepa (onion)>Zea mays.
The label’s recommendation of doing a bioassay is very useful. Many factors can affect herbicide carryover and every case is different. Low soil moisture might not permit appropriate soil conditions for an efficient microbiological and chemical degradation of imidazolinone herbicides³. Other elements to consider are the soil texture, pH, soil temperature, microbial activity, photodegradation, previous crop, water applied after application of the herbicide, soil tillage intensity. The combinations of these elements could contribute to dissipation of these products. Also, having this information would help us determine the best IPM strategy.
 



References:

1. Greenland RG. Injury to vegetable crops from herbicides applied in previous years. Weed Technology. 2003;17(1):73-78. doi:10.1614/0890-037X(2003)017[0073:ITVCFH]2.0.CO;2
2. Herbicide Carryover Study, 1997 Richard Greenland
3. Evaluation of Pre and Post Emergence Herbicides on Yield Contributing and Quality Characters in Onion cv. N-53 M. Venkateswara Reddy* , K. Umajyothi, P. Syam Sundar Reddy and K. Sasikala
4. Efficacy of imidazolinone herbicides applied to imidazolinoneresistant maize and their carryover effect on rotational crops C. Alister1 , M. Kogan

Resistant varieties are crucial for effective insect pest management, more specifically in organic crop production. When growing crops organically, resistant varieties should be the first line of defense against insect pests. Compared to nonresistant varieties, insect-resistant crop varieties are less vulnerable to insect attacks or yield well despite insect attacks. 

Resistance in crops can be expressed through different mechanisms, including antibiosis, antixenosis, and tolerance. For the antibiosis mechanism, the resistant crops act on the life history traits of the pest, such as affecting fecundity, growth, development, and survival. Thus, this reduces the pest population. On the other hand, antixenosis is characterized by the ability of the crop to affect the behavior of the pest by deterring it after initial tasting or probing, which is non-preference. For the tolerance mechanism, however, the crops can sustain a high density of pests and still maintain their yield potential. In most cases, the overall resistance of a variety is a combination of all these three mechanisms. 

There is also a type of resistance called phenological resistance, field resistance, or pseudo-resistance, which is when the crop appears to be resistant because it is at a less susceptible stage when a pest attack occurs. This may happen either because of the rate of development of the crop or due to management practices implemented, causing the crop to escape the pest pressure. Thus, the implementation of proper agronomic practices plays a significant role in improving the ability of crops to resist pest attacks. 

In certain scenarios, additional insect control measures such as the use of bioinsecticides (in organic production) and natural enemies may be necessary in conjunction with an insect-resistant variety for comprehensive pest control. However, even in cases where additional control measures are required, the cultivation of a resistant variety can significantly delay and reduce the amount of insecticide applications needed. This underscores the economic, ecological, and environmental benefits of using insect-resistant crop varieties. 

Previous studies conducted at Yuma Agricultural Center demonstrated that resistant lettuce varieties can provide suitable control of aphids affecting lettuce in the region. Therefore, it is very important to consider cultivating insect-resistant lettuce varieties for suitable management of aphids in organic lettuce production since biological insecticide options are currently limited. My lab is planning to evaluate several lettuce varieties to determine lettuce varieties that carry resistance against aphids in an attempt to generate applicable information on varieties that could fit well within the production practices and environmental conditions of Arizona’s vegetable-growing regions. 

Area wide Insect Trapping Network   VegIPM Update, Vol. 15, No. 21, Oct 16, 2024

Results of pheromone and sticky trap catches can be viewed here.

Corn earworm: CEW moth counts increased in most traps; about average for early October.

Beet armyworm: Trap counts increased in most areas last week, and about average for early October.

Cabbage looper: Cabbage looper trap counts still most locations, below average for this time of the season.

Diamondback moth: Adult activity increasing, particularly in the Yuma and Dome Valleys.  Above average for this time of season.

Whitefly: Adult movement increasing Tacna, Dome Valley and Yuma Valley; overall about average for early October.

Thrips: Thrips adult movement picking up in all areas, overall activity above average.

Aphids: Winged aphids beginning to show up the north Yuma and Gila Valleys; about average for early October.

Leaf miners: Adult activity light in all areas, about average for this time of season