There will be an opportunity to get CA DPR Laws and Regulations CEUs before the season ramps up.

4.0 CA DPR Laws and Regs + 0.5 Other CEUs have been requested, pending approval. 4.5 AZ CEUs requested, pending approval.

Join us on Wednesday, August 26th, 10:30a-3:30p, at the Yuma Agricultural Center. Lunch will be provided. RSVP to maceyw@arizona.edu

Few things in Yuma County are more frustrating than watching lettuce wilt in the field. Even the best-managed fields are not immune to the many destructive soilborne pathogens capable of causing significant economic losses in lettuce production fields. It is an unfortunate reality that Fusarium wilt, corky root, bottom rot, and lettuce drop are encountered and require management nearly every season in Yuma to produce a healthy crop. Although advances in resistant cultivars and integrated disease management have improved our ability to reduce some of the crop losses to these diseases, management options for soilborne diseases, in general, remain severely limited and often are ineffective once symptoms progress to root or crown rot and irreversible wilting. Strong, durable genetic resistance remains the strategic priority and most viable option when it comes to the management of these pathogens.

To select the right genetic resistance package, accurate field history and proper diagnosis of the cause of plant health challenges on that ground is critical. One of the greatest obstacles in plant disease diagnosis is that symptoms alone are rarely what we call pathognomonic, that is, they are seldom unique enough to identify down to a single disease or environmental stressor. Temperature, soil moisture, plant age at time of infection, and varietal susceptibility all influence how diseases and disorders develop and how symptoms are expressed in the plant as a response. As a result, the same pathogen can produce different symptoms under different conditions, while entirely different pathogens may produce nearly identical symptoms. Add to this the complexity of the frequent occurrence of co-infections with multiple pathogens colonizing the same host at the same time and the ability to provide field diagnoses or diagnosis from pictures can quickly become confusing, overwhelming, or downright impossible.

This known variability in symptom expression underscores the importance of confirming a diagnosis rather than relying solely on field observations, especially when the time comes for selecting resistant varieties to be planted in fields with a history of soilborne diseases. Ideally, lettuce shouldn’t be planted on ground that is contaminated with soilborne pathogens, but this isn’t always feasible in our production systems.  

Despite my obligatory warning for the difficulty and unreliability of diagnosing plant diseases and disorders from symptoms alone, I thought it would be a good chance to highlight a few key trends for two infamous diseases that we frequently observed during last season’s atypically warm and rainy weather. I’m talking of course about Fusarium wilt and corky root. My hope is that these observations may prove helpful when scouting wilting fields, explaining your observations, and, importantly, making it easier to document on our submission sheet what you’re seeing in the field if you choose to drop off samples to the clinic for culture testing and confirmation diagnoses. Of course, I remain happy to take a look and sample from the symptomatic field as well. Over the 2025-2026 season the plant clinic received 20 samples culturing positive for Fusarium wilt, 16 for corky root, and 8 for a co-infection with both pathogens simultaneously.  

In the field, the two diseases often appear indistinguishable from aboveground symptoms alone. Both commonly produce wilting, which may be more pronounced particularly during the heat of the day then seemingly “recover” in the evenings. Plants often lose their characteristic healthy, green waxy luster and instead develop a dull, matted appearance that is more apparent when compared with neighboring healthy plants or fields and as the wilting intensifies. As the diseases progress, the outermost leaves surrounding the crown may rot and collapse, further contributing to the appearance of a sick and declining plant. At this point, secondary microorganisms of both the bacterial and fungal kind may set in and produce signs of growth as they feed on the dead tissue, potentially appearing to be related to the cause of the disease and complicating diagnosis.

Below ground, corky root primarily affects the outermost tissues of the root system, causing feeder roots and the taproot to develop yellow to brown (sometime with specks of green) lesions that eventually crack and become roughened. To me, severely affected roots appear as if they are covered in mesquite bark. In contrast, Fusarium wilt primarily invades the plant's vascular system. Although root deterioration may occur (especially on small feeder roots), the hallmark of Fusarium wilt is vascular discoloration within the taproot and crown, where the water-conducting xylem tissues become tan to reddish-brown. These belowground differences, together with laboratory culture or other laboratory testing, provide the evidence needed to confidently distinguish between the two diseases.

Figure 1: Symptoms on a sample culturing positive for Fusarium wilt alone. Left.) outside view of the root system mostly absent of any rot symptoms, and right.) dissected root showing interior vascular discoloration. Note the bright white cortical tissue between the vascular tissue (center) and epidermis (outermost layer of cells, the “skin”) and near absence of overall rot of inner taproot tissue that is commonly observed with other soilborne diseases. The vascular system appears reddish due to the colonization with Fusarium oxysporum f. sp. lactucae.

Figure 3: Symptoms on a sample co-infected with Fusarium wilt and corky root. Left.) outside view of the root system showing significant barky lesions, and right.) dissected root shows a barky vascular dry rot towards the tip of the taproot with reddish-brown discoloration moving up towards the crown. 

The images above are from some of the more diagnostically clear samples we have received in the clinic. As mentioned earlier, timing of infection, local microclimate, and varietal genetics can play a huge role in defining how a plant expresses disease symptoms, so these observations of textbook symptomology may not be as clear in every field with wilting that you manage. For these reasons, laboratory testing remains the gold standard for disease diagnosis. Submission of symptomatic plants to the diagnostic clinic allows for pathogen isolation and culture, providing definitive identification that can distinguish between diseases with similar symptoms. Accurate diagnosis is the foundation of effective disease management, ensuring that control measures are directed at the true causal agent rather than its look-alikes. The Yuma Plant Health Clinic is here to help to that end.

If you have any concerns regarding the health of your plants/crops please consider submitting samples to the Yuma Plant Health Clinic for diagnostic service or booking a field visit with me:

Christopher Detranaltes, Ph.D.
Cooperative Extension – Yuma County
Email: cdetranaltes@arizona.edu
Cell: 602-689-7328
6425 W 8th St Yuma, Arizona 85364 – Room 109

VegIPM Update Vol. 17, Num. 7
April 1, 2026

Results of pheromone and sticky trap catches below!!

Corn earworm:  CEW numbers remained high over the last two weeks; numbers remain high particularly in Welton and Tacna areas. Above average for this time of season.

Beet armyworm: BAW activity remained steady over the last two weeks. Above average for this time of the season. Historically BAW numbers are expected to continue increasing over the next few weeks.

Cabbage looper: Cabbage looper moth activity decreased over the last two weeks but remains higher than average for late-March. Based on historic data, expect to see high numbers through the end of the produce season.     

Diamondback moth: Adult activity increased across most locations. High numbers seen in Yuma Valley.  About average for this time of the year.   

Whitefly: Adult activity increased across all locations, about average for this time of year. Expect numbers to continue increasing as temperatures warm up.

Thrips: Over the last two weeks, thrips adult activity continued to increase across locations and reached season highs. Higher than average for this time of year.  Expect numbers to continue increasing as temperatures warm up

Aphids: Winged aphid numbers continue to decrease at most sites in the last two weeks. At our efficacy trials at YAC, most aphids died due to the high temperatures. below average for this time of the year.

Leafminers: Adult leafminer activity remains steady, high numbers continue to be seen in Yuma Valley, about average for this time of the season.