Once again, we are experiencing a prolonged heat wave in the Salinas Valley. Maximum air temperature in the King City area reached 112 °F earlier in the week (Fig. 1). Recent maximum air temperatures in South Salinas have been far greater than the average temperatures recorded for the same period during previous years (2015 through 2019).
Although this heat wave will probably wane in the next several days, the central coast region will likely experience periods of record setting temperatures in the future. There are several concerns about how prolonged elevated temperatures affect cool season vegetables. Heat can cause immediate damage to plant tissue when temperatures of the plant surfaces become too high and cause cells to die (Fig. 2). In addition, sustained high temperatures can affect plant growth and development. For instance, in lettuce damage can vary from obvious burning on the edges of leaves from too much heat load (Fig. 3) to more physiological issues that result in poor head formation in iceberg (e.g. puffy heads). In broccoli, if heat damage occurs when heads are forming, it can result in uneven bead sizes when the head matures (Fig. 4). Excessive heat can result in wilting in cauliflower (Fig. 5) during high temperatures and expose curds to sunburning or cause discoloring (Fig. 6.) In the past two years, we have observed that excessive heat can stress lettuce plants and make them more susceptible to infection with Pythium Wilt (Pythium uncinulatum). That was particularly evident in the 2020 heat spells. If there is inoculum Pythium Wilt in the soil, stress caused by heat on the plant can set off infection (Fig. 7).
A previous article presented strategies for maximizing evapotranspiration rates to keep crops cool. Evapotranspiration (ET) is the process in which liquid water vaporizes from plant leaves and moist soil surfaces and is lost to the surrounding air. As liquid water vaporizes, heat is also lost from the surfaces of leaves and soil and from the surrounding air, which cools the crop. Assuring that crops have adequate soil moisture during the hottest period of the day (generally 11 am to 4 pm) can keep plants as cool as possible. Insufficient moisture to meet crop water requirements can result in stomates of the leaves closing and decrease transpiration rates. Limiting transpiration would raise leaf temperatures, potentially to temperatures greater than the surrounding air.
Hence, a good strategy to prevent heat damage to vegetable crops is to water fields that have not been recently irrigated. Also, keep in mind that during the last few days daily reference ET increased substantially due to the high air temperatures and so more water is needed than normal to replace the amount of moisture that crops transpiration. In South Salinas, for example, the CIMIS station showed that daily reference ET increased from 0.18 inches per day in late August to 0.25 inches per day during the heat wave, approximately a 40% increase in water demand (Fig. 8).
Irrigations do not need to be very long, as much as they should supply the crop with enough water to refill the soil profile to the depth of the root zone. Irrigating more frequently for less time would be a better strategy than irrigating less frequently for more time, since the soil has a limited capacity to store water in the root zone. Over-saturating the soil during high soil temperatures through heavy irrigations could worsen infections from soil-borne pathogens.
The CropManage online decision support tool can assist with determining the amount of water to apply and frequency to irrigate for most vegetable crops produced in the Salinas Valley. The software allows one to customize the recommendations for the development stage of the crop, soil type, and irrigation system characteristics.
Finally, for crops irrigated by sprinklers, short irrigations during the hottest time of the day can reduce air temperatures. This might be a good strategy for vegetables that are in a stage of development that is very susceptible for heat damage, such as cauliflower close to harvest.
- Author: Richard Smith, UCCE Monterey
- Author: J.P. Dundore Arias, CSU Monterey Bay
- Author: Michael Cahn, UCCE Monterey
Pythium wilt of lettuce (Pythium uncinulatum) continued to be a significant production problem in lettuce fields in 2021. The levels of infection were not as extensive or severe as in 2020, but the damage caused by this disease was nonetheless problematic and serious in many fields. Pythium wilt was more frequently observed towards the end of the production season, and the development of noticeable foliar symptoms and severity of the disease appears to increase with higher temperatures. As an example, we observed growers successfully growing spring lettuce crops on blocks that were wiped out with Pythium wilt the prior fall, likely due to the lack of conducive conditions for the disease to develop. Given the rapid rise of Pythium wilt as a serious soilborne disease of lettuce in the Salinas Valley, there is a great need to better understand its biology and epidemiology to help determine the most promising means of managing it: cultural, chemical and/or varietal. In this write up we report on studies conducted in 2021 that included 1) evaluations of applications of fungicides and biologicals, 2) evaluation of effect of irrigation management on the incidence of the disease and 3) observations of varietal tolerance. Bottom line is that fungicides and biological control of Pythium wilt was measurable but limited in our studies. In the irrigation trials, we did not see higher incidence of Pythium wilt in the plots receiving greater quantities of irrigation water. There is good varietal tolerance to Pythium wilt in some commonly used lettuce varieties and to-date, this looks to be the most promising method for effectively reducing Pythium wilt losses.
Fungicide and Biologicals Evaluations: Twelve trials were conducted to test the efficacy of fungicides and biologicals for controlling Pythium wilt of lettuce. In ten trials, materials were applied over-the-top of the lettuce and incorporated into the soil by subsequent sprinkler irrigation applied within two days of application; in two trials, Ridomil Gold was injected into the drip system during an irrigation (see Table 1 for details). Top of the label rates of the fungicides and biologicals were used for each application to increase the chances of obtaining a signal from the materials. Fungicides tested in the over-the-top trials included Ridomil Gold, Previcur, Alliette and Ranman. Biologicals tested were Minuet (Bacillus subtilis) and LifeGuard (Bacillus mycoides). Over-the-top applications we made at-planting, thinning and/or at the rosette stage, but applications at each of these timings was not always possible in each trial and Table 1 shows the application timings for specific trials.
Planting dates for the trials ranged from June 12 to August 25. Trials were evaluated at or near harvest by counting all plants in each plot and then counting plants infected with Pythium wilt to get the percent infected plants. At times plots were also infected with Sclerotinia and a vascular wilt, and plants infected with these diseases were kept separate from the Pythium wilt totals. Trials planted before late July had levels of Pythium wilt that were too low to effectively evaluate (data not shown). However, trials planted in late July or after had greater incidence of disease. A confounding factor that affected the level of infection in later trials (Nos. 8, 9 and 10) was that by chance, the trials were located in fields planted with the green leaf variety Green Teen which appears to have significant tolerance to Pythium wilt and thus, these trials had little incidence of Pythium wilt (data not shown). Trial No. 7 provided the most useful data to evaluate the efficacy of over-the-top applications. The materials were applied at all three application timings and, on three evaluation dates, there was a trend indicating a lower percent of infected plants in each fungicide and biological treatment (Table 2). Although the trend was weak, it was consistent over each evaluation date. The data indicate a measurable but limited level of efficacy from over-the-top applications of these materials. Both Ridomil injection trials (Nos. 11 & 12) had significantly fewer infected plants in the Ridomil treated plots at harvest. Both fields had high levels of infection by Pythium wilt, and the reduction in infected plants was measurable but limited.
In summary, there was a modest level of control of Pythium wilt with the applied fungicides and biologicals. Injection into the drip system appeared more effective than over-the-top applications. It appears that evaluating a more effective means of applying fungicides and biologicals may be useful to see if better control may be achievable, especially in fields with history of high incidence of the diseases and during the fall lettuce crops when warm temperatures are expected. However, the challenge remains getting the material to move throughout the volume of soil occupied by the root system. This is important because infections with Pythium wilt can start on finer lateral roots or deeper in the soil and come up the tap root. Getting effective concentrations of a fungicide or biological material to the whole root system becomes a great challenge. Similarly, determining the right time of application requires further investigation.
Irrigation Evaluations: Pythium wilt is a water mold that can produce mobile spores. Excess water in the soil is thought to favor the development and spread of this organism. The goal of these trials was to evaluate if the volume of applied water might affect the incidence of Pythium wilt on lettuce. Four trials were conducted evaluating the impact of the amount of irrigation water applied during the crop cycle on the incidence of Pythium wilt. Trials were conducted in cooperating growers' fields. Irrigation regimes were established that compared 100% of crop ET (as calculated by CropManage) with 150 and 200% of crop ET. All trials were drip irrigated and the higher amounts of irrigation were applied in trials 1, 2 and 4 by using with separate manifolds equipped with a flow meter to measure the quantity of water applied. Trial 3 simply had two drip lines installed in the 200% ET treatment. At harvest, the plots were evaluated for the percent wilted plants (no. wilted plants/no. total plants). A sample of 10 plants from each plot was further evaluated to confirm if the cause of foliar wilting was Pythium wilt, Sclerotinia or a vascular wilt.
There were low levels of wilted plants in the first two trials (Table 3). In the later trials (3 & 4) there was significant wilting in the plots. However, there was no significant difference in the level of lettuce plants infected with Pythium wilt among the irrigation treatments in these trials.
Observations on Varietal Tolerance to Pythium Wilt:Formal variety trials were not conducted in 2021, but as opportunities to observe the response of varieties to Pythium wilt presented themselves, we made note of the level of infection in the varieties. One particularly dramatic example is shown in Table 4. At this site, there were two planting in which full beds (1 – 9 beds) of various varieties were planted in a field significantly infected with Pythium wilt. The percent of wilted plants in the different varieties, which ranged from 1.5 to 47.6% of total plants. The varieties Momentus and Copious had the lowest number of wilting plants and the lowest percent of Pythium infected plants, indicating good tolerance to this disease. In these two plantings, there were also plants infected with Sclerotinia and a vascular wilt resembling Fusarium wilt, though pathogen identity was not confirmed beyond visual observations. In some cases, it appeared that plants had both vascular wilt and Pythium wilt. These results indicate the co-occurrence of Pythium wilt and other soilborne diseases and raises the question whether they may interact while infecting a susceptible host. Clearly, more research is needed to better understand varietal tolerance of Pythium wilt and other associated soilborne diseases. In the end, varietal tolerance showed the greatest reduction in damage caused by Pythium wilt. The good news is that this tolerance is present in currently used breeding lines and hopefully can be quickly incorporated into other commonly used lettuce types.
Acknowledgements: We are grateful to the many cooperating growers and PCA's that helped us with these trials. We thank the California Leafy Greens Research Board for funding and thank the following research assistants for their help on these evaluations: Noemi Larios, Tom Lockhart, Tricia Love, Carlos Rodriguez Lopez and Yulissa Soto.