Feb 18, 2026
A note on the current variation characterized in the pathogen causing Fusarium wilt of lettuce
Fusarium wilt of lettuce, caused by Fusarium oxysporum f. sp. lactucae (Fol), continues to be one of the most economically significant soilborne diseases affecting lettuce production. To date the most successful strategy for disease management, outside of preventing the introduction of the pathogen into new fields, has been the development and deployment of Fol resistant lettuce varieties. Resistant varieties carry genes which allow the lettuce to impede or respond to the pathogenic invasion through a defense response. The result of this interaction is often tolerance to the presence of the Fol, meaning genetically resistant varieties are still capable of getting infected but show less damage, form marketable heads more often, and may even slow the reproduction of the pathogen in the field.
Unfortunately, one size does not fit all. Each Fol resistance gene is not expected to be effective at defending against every isolate of Fol. When a new resistance gene is introduced into crop varieties, it imposes a strong selection pressure on Fol populations, favoring isolates that can overcome or evade the host’s genetic resistance. Put simply, isolates that are controlled by the resistance don’t reproduce, while random changes can allow other isolates to avoid detection and reproduce a lot. In some cases, change in just a single gene in the pathogen can cause a breakdown in varietal resistance and since fungi can mutate and reproduce much faster than we can breed new lettuce varieties, we are often on the losing end of this “arms race”. The progeny of isolates that overcome the resistance increase in frequency and eventually become predominant in the local pathogen population, ultimately resulting in declining performance of the resistant variety in the field.
For plant pathologists, geneticists, and breeders we need to have a way to describe and report this variation in response to different resistance genes. Hence breaking pathogens into “races” (also referred to as physiological races or pathotypes) is the practical way we capture the biologically meaningful variation within a single species of pathogen with regards to their ability to infect different varieties. A race is typically defined by what specific varieties it is able to cause disease on, and the panel of varieties with known resistance we use to determine races of a pathogen are called differentials. These differentials are a product of academic and industry breeding efforts to try to capture the most relevant patterns of disease susceptibility and pathogenicity that exists across commercial production environments.
Four races of Fol (designated 1, 2, 3, and 4) have been identified globally based on the ability to infect seven varieties. Race 1 was the first to be characterized and has been reported in the major lettuce-producing regions of Japan, Taiwan, the United States, and parts of Europe. It is as of yet the only race to have been reported in the United States. Resistance to race 1 has been successfully incorporated into many commercial lettuce varieties, and for years this provided control in affected areas.
However, in response to reports from Californian lettuce growers regarding abnormal disease patterns, namely increased susceptibility in race 1-resistant cultivars and reduced severity in susceptible cultivars, researchers also identified additional novel variant samples of Fol from California lettuce fields. Among them was the variant designated CR4 (named for its inability to cause disease on the differential variety ‘Costa Rica 4’) which was consistently able to exhibit a strong and unique pathogenicity pattern on the seven resistance differentials not matching any of the four previously known races. The researchers who discovered and characterized this new CR4 variant are now in the process of fulfilling the criteria necessary to have it internationally recognized as a new race. Until then, it is being referred to as a variant. Initially this variant was isolated from samples from coastal lettuce fields, however, occurrences across multiple fields and two non-adjoining counties (Monterey and Santa Barbara) indicated that it may have already spread across the major lettuce production regions of California and even Arizona as well. Since its discovery, CR4 was confirmed to be present in lettuce samples from Arizona by the same group who initially discovered it in California.
This lettuce season we’ve received some feedback concerning the possibility of race 1 resistance breakdown in lettuce in Arizona lettuce fields. It is not yet known in all of these cases if the infection of race 1-resistant material is due to CR4 variants that are evading race-1 resistance or if the abnormally warm and wet lettuce season made race-1 resistance look less effective than in past years, both of which are possible explanations this season. Determining this answer is made complicated by the pathotyping processes currently used to determine an Fol isolate’s race. It requires the precise inoculation of several different varieties under tightly controlled environmental conditions to see on which varieties the strain can and cannot induce disease. With Fol, this is further complicated by the need to designate intermediate reactions between complete susceptibility and resistance, which symptomatically can be difficult to reliably distinguish across pathotyping experiments.
However, researchers at the University of California led by Dr. Alexander Putman and in collaboration with the USDA-ARS are working on developing a DNA-based test that will be able to rapidly detect the Fol CR4 variant. The process bypasses the need for artificial inoculations and can be achieved much faster and more reliably than through inoculation-based methods. Updates on this exciting work will be featured in the plant pathology breakout session during this year’s Southwest Ag Summit. Be sure to drop by room AS112 on February 19th, from 1:30 – 3:30 pm to hear all about it!
Until then, while the CR4 variant does show the ability to evade the race 1 resistance bred into some varieties, it does not appear to be able to infect all lettuce varieties equally. In fact, it appears some varieties that have historically been susceptible to race 1 show increased resistance to the new variant, which is a promising sign that genetic resistance will remain available despite this pathogen’s shifting nature. The development of this DNA-based test will allow us to gauge the relative populations of each race across Yuma County and to tailor varietal recommendations depending on which race or variant is predominant in the field.
Recommended reading:
Nayak, Santosh et al. “Detection of Novel Pathogenic Variants of Fusarium Oxysporum f. Sp. Lactucae in California.” Plant pathology 74.2 (2025): 295–307.
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
