Clubroot disease can be a serious production issue for broccoli, cauliflower, and other brassicas in the Salinas Valley. The disease is caused by a unique organism (Plasmodiophora brassicae) that is closely related to ciliate protozoans but is classified in its own taxonomic group. It survives over 20 years as resting spores in the soil that are released as the clubbed root tissue decays. At temperatures above 65 °F, the resting spores release zoospores that swim to host plant roots and infect through root hairs. Once inside the plant, the organism grows into a large multinucleate plasmodium (a multinucleate mass of protoplasm) which stimulates changes in the plant hormones, resulting in enlarged root cells and the characteristic clubbing of the roots (See photo Below). Root infections by the clubroot pathogen can occur in both acid and alkaline soils; however, acidic soil conditions favor the development of the root symptoms. In addition to the main brassica crops, Plasmodiophora can infect arugula, radish, mustard cover crops, and weeds such as shepherd’s purse and even some grasses. Plants that develop severe root swellings will exhibit above ground symptoms (See Photo Above) indicative of non-functioning root systems, which includes yellowing, wilting, poor growth and stunting, drying and death of lower leaves, and eventual plant death.
Clubroot in the Salinas Valley is mostly controlled by maintaining soil pH above 7.2 to 7.3 by liming. The high pH does not kill the pathogen but inhibits the formation of the root clubs. Soils where control of clubroot by liming is achieved are called “responsive” soils. However, soils where liming is less effective are called “unresponsive” soils.
In 2020 we had calls regarding the incidence of clubroot on brassicas. In each situation the grower/PCA had soil lab results that indicated that the soil pH was greater than 7.2. To investigate this situation, a small study was conducted. At three fields soil was collected from symptomatic and asymptomatic areas of the crop and soil pH was determined using a pH meter at the UCCE or UC Davis Analytical Lab. The results shown in Table 1 indicate that clubroot was more severe in soils with lower soil pH levels. These findings are consistent with what we know about clubroot, that higher pH soils should have less concern with this disease.
Table 1. Three evaluations of soil pH in clubroot affected fields
So why did clubroot occur in soils that had test pH values greater than 7.2? It is important to keep in mind that soils have a great deal of inherent variability. The goal is to determine if the soil pH for a 5 or 10 acre field is ≥7.2. This is typically done by collecting 15 – 20 soil cores from various parts of the field and mixing them together as a composite sample. However, if sample collection by chance missed areas of lower soil pH, the lab results may be skewed to represent areas of the field that had relatively higher pH values. If this is the case, such a sample could have an artificially high pH (greater than 7.2) while some parts of the field may have a lower pH value. One way to have greater confidence in the soil pH is to collect more soil cores in fields where clubroot disease has been noted in the past.
At present, we have not seen evidence in Monterey County that soils are unresponsive to liming or that the liming treatment is failing to control clubroot, given variability in soil pH and pH testing. In our intensive vegetable production system, soil pH tends to decrease over time through the use of ammonium fertilizers. The loss of calcium, magnesium and potassium from crop removal and leaching can also contribute to lower soil pH on lighter soils. Given the longevity of clubroot resting spores in the soil, it is important to maintain a liming program to assure that soil pHs are above 7.2 to 7.3 to thoroughly suppress clubroot throughout the field. – By Richard Smith, UCCE Vegetable Crops & Weed Science Farm Advisor, and Steve Koike, TriCal Diagnostics
Fusarium is a soil borne fungus that invades the root system. Once infected, the entire root will turn reddish-purple to grayish-black. This is in contrast to a Phytophthora infection which, in many cases, will attack only the feeder roots, but when larger roots are infected, only the inner bark is decayed and it does not discolor the wood. In addition, when observing the cross section of a dry root rot infected trunk, a grayishbrown discoloration in the wood tissue can be observed.
Gloves, N95 respirators, coveralls and other gear that workers wear to protect themselves from COVID-19, pesticides, dust and other health hazards are in short supply as priority is given to health care workers during the pandemic.
Respirators: If you can’t find the respirator required on the label, Blecker said, “Use an alternative, more-protective respirator. For example, if an N95 is required, you can use a half-mask with N95 particulate filters; these can be stand-alone filters or ones that attach to an organic vapor cartridge. You could also use a different pesticide that doesn’t require a respirator. Consult with your PCA (pest control adviser) for options.”
“DANGER,” or when applying by backpack or airblast. “Coveralls can be made out of high-density polyethylene fibers (Tyvek and other brands), which are disposable, or cotton, which are reusable,” Brim-DeForest said. “If reusable coveralls are worn, the employer must ensure employees are provided clean coveralls.”