Strategies to Control Pythium Wilt of Lettuce

Author: Richard Smith and JP Dundore-Arias

Farm Advisor, UCCE Monterey; and Plant Pathology Professor, Cal State Monterey Bay

In 2020 Pythium wilt of lettuce (Pythium uncinulatum) caused up to 100% yield loss in some fields in the Salinas Valley. Although, Pythium wilt has been present in the Salinas Valley for nearly a decade, the widespread severity of the outbreaks this year were devastating and unprecedented. 

Symptoms of the above ground parts of the plant include stunting, yellowing, and wilting of the outer leaves and eventual death (Photo 1). Sometimes the plants have a characteristic look where the younger leaves remain upright, but the older leaves wilt down (Photo 2). Upon examination, the roots of Pythium wilt infected plants, exhibit rot on the feeder (Photo 3) and/or the tap roots (Photo 4). The crown of the plant does not rot (however, in advanced infections the whole length of the root may be rotted) and this is one way to distinguish Pythium wilt from Sclerotinia. For instance, when you gently tug on wilting plants, Pythium infected plants do not break off at the soil line, whereas plants infected with Sclerotinia readily break off at the soil line and exhibit characteristic white mycelia and black sclerotia. The presence of rot on the roots distinguishes Pythium wilt from Verticillium and Fusarium wilts which cause internal vascular discoloration, but not external rot.  If you are unsure about the cause of wilting/dying plants, it is best to have them tested.

Pythium wilt infects lettuce roots with swimming spores (zoospores) that move to the roots in soil water films. Additionally, it produces a second type of spore (oospore) that allow long-term survival in the soil. Moreover, Pythium species are generally known as good soil saprophytes, characterized for their ability to grow and survive in the soil even in the absence of a host plant by living on organic matter. Previous studies have reported P. uncinulatum is a pathogen of lettuce and does not cause disease on other vegetable crops. However, it remains unknown whether rotations with other crops may contribute to a build-up of the pathogen in the soil.

Devastating outbreaks of Pythium wilt mostly occurred on the east side of the Salinas Valley north of Gonzales on decomposed granite soils, but outbreaks also occurred in clay loam and sandy loam soils along the river. Given that Pythium wilt is a water mold, it was often most severe in wetter parts of the field with slower drainage, such as the bottom end of the field. However, in severe infestations Pythium wilt occurred the entire length of the field, particularly on susceptible varieties.

It is not clear what might have triggered the disease outbreaks, but widespread infections occurred following heat spells in mid-August and early-September. It is not clear why the heat may have triggered the outbreaks, but it is possible that additional irrigation water applied to cope with the heat could have imposed additional stress on plants with already unhealthy root systems. However, this explanation is not entirely satisfactory as smaller outbreaks were also observed in June and July prior to heat spells.

Pythium wilt infections frequently occurred in association with Impatiens Necrotic Spot Virus (INSV) infections. In trial 5, 8.4% of the plants had Pythium wilt and nearly all of them,7.9%, were also infected with INSV (Table 2, Photo 5). However, this was not always the case; in trial 4, plants with symptoms of Pythium wilt occurred on 22.6% of the plants, but only 0.8% were infected with INSV. Coinfection with Sclerotinia, Fusarium and Verticillium also commonly occurred.

Fungicide Trials

In late September, we conducted Pythium wilt control trials in commercial fields. Each trial consisted of areas treated with 2-pints/A of Ridomil Gold which was compared with an untreated control. Ridomil Gold is registered for use on lettuce for at-planting soil applications to control damping off caused by Pythium sp. These trials were initiated too late in the planting season to include at-planting treatments, so Ridomil was applied at the following timings: 2 true leaf stage (one trial), at thinning (2 trials) and at the rosette stage (3 trials) (See Table 1 for more details).

In trials 2 and 3 we measured a significant reduction in the number of plants per plot that showed symptoms of Pythium wilt (Tables 3 and 4); no significant reduction in infected lettuce plants was observed in the other four trials. These trials showed that Ridomil applied at thinning and at the rosette stages could provide limited control of Pythium wilt. The best control was observed in the at-thinning application in Trial 3. It is not clear why Ridomil did not provide better control in the other four trials. We did not have the opportunity to examine at-planting applications in this year's trials, but they will be examined in 2021. Pythium wilt affects feeder roots on lettuce plants which are higher up in the soil profile, but it has also been commonly observed to attack that tap root of lettuce indicating that the infections can start deeper in the soil profile. Ridomil is mobile in soil which should allow it to distribute throughout the root zone. Questions remain regarding improving the efficacy of Ridomil: what are the effective rates, what is the most efficacious timing of application and can split applications improve efficacy?

Varietal Resistance

There were significant differences in the susceptibility of varieties to Pythium wilt. Table 5 shows the results of ratings of varieties in several commercial production fields and one seed company variety trial. The evaluations were made at or near crop maturity and the lower numbers indicate less infection.  None of the varieties were completely resistant to Pythium wilt, but several had greater tolerance.  In general, the red leaf types were more tolerant than green leaf types, but there were some green leaf types that were tolerant as well (Photo 4). There were several head types that were also tolerant, but no tolerant romaine lettuce types were observed.

It is unclear if Pythium wilt will be a recurring threat to lettuce production in 2021. As mentioned, in 2020 the widespread incidence of the disease occurred in late summer. Pythium wilt was observed to a lesser extent in the spring of 2020 and the question whether it will cause widespread infection in the cooler months of 2021 is unknown. Another remaining question is whether disease occurrence and first appearance of symptoms might be accelerated, or aggravated, in soils where high disease severity was observed in 2020. The disease was mostly found north of Gonzales and there is a question whether it will extend its range in the future. There is a great deal that we do not know about managing Pythium wilt of lettuce, and we'll have to be watchful in 2021 using the few lessons and tools from 2020 that we gained.

Photo 1. Healthy plant on left, Pythium wilt infected plant on right.

Photo 2. Younger leaves remain upright while older leaves wilt down.

Photo 3. Pythium infection on fine roots. 

Photo 4. Pythium infection on tap root.

Photo 5. Pythium wilt infected plant with lesions from INSV evident on older leaves.

Photo 6. Tolerant variety on the left and susceptible on the right.

Table 1. Trial details of Pythium wilt (PW) control trials.

1 – Spray = material sprayed over the top of the plants and incorporated by subsequent sprinkler irrigation (within 24 hrs)

2 – SL = seed lines

Table 2. Percent of plants infected with Pythium wilt and other diseases

Table 3. Number of plants infected with Pythium wilt in each plot in trials 1 and 2 on evaluation dates.

Table 4. Number of plants infected with Pythium wilt in each plot in trials 3-6 on evaluation dates.

Table 5. Informal evaluations of varieties for Pythium wilt of lettuce.

* Scale for Pythium wilt infection: 0 = plants all healthy to 10 plants all dead.

Up-coming on-line meeting on November 10th, 2020 from 12:50pm- 4:00pm. 

This will be a Zoom event. 

If interested in participating, please register at:

http://ucanr.edu/survey/survey.cfm?surveynumber=32220 

Agenda for this meeting is below. 

Up-coming on line meeting on December 1st, 2020 from 8:50am-11:30am

This will be a Zoom event. 

If interested in participating, please register at:

https://ucanr.edu/survey/survey.cfm?surveynumber=32220 

Agenda for this meeting is below. 

Authors: Richard Smith, JP Dundore Arias, Daniel Hasegawa and Steve Koike

Farm Advisor, UCCE Monterey; Plant Pathology Professor, Cal State Monterey Bay; Research Entomologist, USDA ARS; Director, TriCal Diagnostics, respectively

In 2020 the incidence of Pythium wilt (caused by Pythium uncinulatum) of lettuce has increased in severity and in the number of affected fields. Pythium infections in lettuce fields have been observed frequently, but not always, occurring with INSV infection. As a result, there has been confusion distinguishing between these two diseases and the role of each of them in causing the problems in fields. In this blog we will discuss these two diseases and explain from our current state of knowledge about the disease dynamics occurring in affected fields.  

 INSV has been a production problem on lettuce in the Salinas and surrounding valleys for a number of years and in 2020 it continues to be a serious production issue. Pythium wilt of lettuce is a relatively new problem and was first discussed in a blog entry in October 2015 by Steve Koike (https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=19327 ). However, in 2019 and 2020 we have seen an increase in the number of acres affected by Pythium wilt as well as the severity within fields. Given that Pythium is a relatively new problem and the extent of the problem suddenly increased, some growers and PCAs are confronting this problem for the first time. To add to the confusion, at times INSV and Pythium infections are occurring together on the same plants which has caused confusion and has led to much speculation about the role of each disease in the observed damage.

Symptoms of INSV

Issues with INSV infections on head and leaf lettuce types are not a new occurrence in the Salinas Valley and many growers and PCAs are familiar with the symptoms and the patterns of infection in the field, especially on romaine. In general, INSV on lettuce causes characteristic patterns of chlorosis and necrosis on the inner leaves of the plant, as well as significant stunting (Photo 1). However, INSV can cause significant necrosis and lesions on and at the base of the ribs of lettuce plants (Photo 2). It should be mentioned that Tomato Bushy stunt virus (TBSV) can cause symptoms that can be confused with INSV and Pythium wilt; however, lettuce dieback symptoms are always seen on the outer, older leaves and the TBSV pathogen is commonly restricted to low-lying areas along the river. In addition, head lettuce varieties and some romaine varieties are resistant to this virus. When in doubt, it is important to have a sample tested. That said, INSV is the overwhelming virus issue facing growers and PCAs in 2020.

Photo 1. Common symptoms of INSV on romaine lettuce.

Photos 2a and 2b. Moderate to severe symptoms of INSV on ribs of romaine.

Viral vs. Fungal Symptoms

One important detail about lettuce plants infected with only INSV is that they do not exhibit wilting of the outer leaves of the plant or show root rot or root discoloration. This is important to note because in 2019 and 2020 we have visited many fields where the plants exhibit symptoms of INSV and have wilting older leaves. In these situations,  the roots and crowns of the plants should be examined for symptoms caused by soilborne pathogens such as the wilt pathogens (Fusarium and Verticillium), Sclerotinia, and Pythium. Fusarium and Verticillium do not cause rot on the fine roots or crown. However, they always cause characteristic vascular discoloration in the taproot and crown of the plant. Distinguishing these two pathogens without a laboratory evaluation is not advised, but in general, Fusarium occurs earlier in the crop cycle and often causes a red-to-brown discoloration internally along the taproot and at the base of the crown. Symptoms of Verticillium on the above ground parts of head lettuce become obvious close to harvest; the taproot and crown tissue of infected plants have dark brown-to-black discolorations. Plants with INSV can also be infected with Sclerotinia (S. minor) which is recognized by the characteristic rotting of the crown tissue of the plant and the presence of white, cottony growth and small blacksclerotia (Photo 3). Plants infected with Sclerotinia easily break off at the soil line when you gently tug on them. However, if the plants do not break off at the soil line and do not show any rot on the crown tissue but do exhibit rot on the fine feeder roots or lower down on the taproot, then Pythium wilt is suspected and can be verified by laboratory evaluation.  

Photo 3. Sclerotinia infection on lettuce. Note that  it infects and rots crown tissue of the plant.

Biology and Symptoms of Pythium

Pythium wilt is caused by the water mold, Pythium uncinulatum. It infects lettuce roots with swimming spores (zoospores) that move to the roots within the water film in the soil. Additionally, it produces a second type of spore (oospore) that allows the pathogen to survive in the soil in the absence of a host plant. Previous studies have reported P. uncinulatum is almost exclusively a pathogen of lettuce and does not cause disease on other vegetable crops. However, it remains unknown whether other crops may contribute to a build-up of the pathogen in the soil. Affected plants will exhibit rotting of the fine and tap roots (Photo 4) and frequently dark discoloration of the inner core of the main root (Photo 5). Symptoms of the above ground parts of the plant include stunting, yellowing, and wilting of the outer leaves and eventual death (Photo 6). Sometimes the plants have a characteristic look where the younger leaves remain upright, but the older leaves are totally wilted down to the soil (Photo 7). This year, we frequently observed fields where plants are infected with Pythium wilt but are also infected with INSV. These mixed infections are confusing and make it more difficult to distinguish what is the cause of the damage. In our experience to date, plants that show foliar symptoms of INSV and that have wilting older leaves are typically infected with both INSV and, in many cases, Pythium wilt. It should be mentioned that we have also observed plants infected with INSV as well as Fusarium.

 The distribution of Pythium wilt in a lettuce field can be variable. Earlier in the summer, fields with this disease typically were infected along the upper or lower ends of the field indicating that the disease may be responding to irrigation or drainage issues. It is possible that there may be a difference in the level of infection between sprinkler and drip irrigated fields, but we cannot say anything definitive at this time. The disease has been found from King City to Castroville. There is a significant difference in the susceptibility of varieties. In fields with multiple leaf type lettuce, we have observed significant differences in susceptibility among varieties with red types being less susceptible (Photo 8). Recently, there have been severe losses in some fields. It is not clear as of this writing, but it is possible that the incidences occurred in response to the heat spells. It is likely that diseased plants were not able to withstand the weather stress due to damaged roots or that extra water applied to address the heat may have stimulated the development of Pythium wilt. Another observation we have made is that at times Pythium mostly infects the fine roots higher up on the root system and in other situations it is more severe at the bottom of the taproot (Photo 9) which may indicate disease initiated farther down on the root system. Given that the disease needs a period of soil saturation for the swimming spores to travel to the roots, issues with soil preparation, drainage and irrigation management may affect the severity of the disease.

 Research Efforts

Daniel Hasegawa is conducting research on the epidemiology and spread of thrips and INSV. JP Dundore Arias is working on a project with the California Leafy Greens Research Board monitoring the occurrence of Pythium wilt in the Salinas Valley. He is also characterizing isolates of this disease to better understand the organism and will be conducting preliminary evaluations of the sensitivity of the organism to fungicides. Given the rapid onset of severe damage of Pythium and the continued severity of INSV, we are trying to better understand these diseases and how they may interact. We are interested in receiving samples of Pythium wilt. Please contact Richard (rifsmith@ucdavis.edu) or JP (jdundorearias@csumb.edu) to submit samples.

Photo 4. Pythium wilt infection of fine lettuce roots.

Photo 5. Pythium wilt infection on lettuce taproot.

Photo 6. Mini romaine infected with Pythium wilt.

Photo 7. Romaine infected with INSV and Pythium wilt. Note that the older leaves are wilted and lay on the ground.

Photos 8a and 8b. Difference in susceptibility of two green leaf lettuces (photo on top) and a green leaf and red leaf lettuce (photo on bottom) to Pythium.

Photos 9a and 9b. Plant with Pythium wilt infection lower down on the tap root (note the plant on the top with healthy fine roots higher up and infected tip of the tap root).

Currently, we are experiencing a prolonged heatwave on the central coast.  Heatwaves have become a recurring phenomenon in recent years, especially in late summer. With thousands of acres of cool season vegetables in the ground, irrigation will be critical for keeping crops cool and for supplying enough moisture to meet their water needs. 

Crops can be kept cool by maximizing evapotranspiration (ET).  As liquid water vaporizes heat is lost from the surfaces of leaves and soil and from the surrounding air, which cools the temperature of the crop.  Under water stress leaf stomates close during the hottest period of the day (11 am to 4 pm) and the temperature of the plant tissue can rise above the temperature of the surrounding air.  If the temperature becomes too great leaves and other plant parts may become scorched (Figure 1).

Figure 1.  Heat damage in broccoli

Since most ranches have a limited number of wells and personnel to irrigate, it is challenging to assure that each field has adequate soil moisture to prevent plants from overheating.   A good strategy is to irrigate just enough to refill the soil profile to the rooting depth of the crop.

To prioritize which fields to irrigate one should consider the water holding capacity and existing level of moisture of the soil, as well as rooting depth and developmental stage of the crop.  For example, a lettuce crop near maturity with a high ET demand, growing on a sandy textured soil that feels dry, should probably be irrigated soon.  A young lettuce crop with a low ET demand, growing on silt loam soil that still feels moist, likely can be irrigated later without suffering heat damage.

Another consideration for prioritizing which fields to irrigate are recent field operations.  A recently transplanted vegetable field may need to be irrigated first but may not need a long irrigation to re-saturate the soil around the roots.  A crop that was recently cultivated may have pruned roots, and therefore may need water soon to prevent wilting under these hot conditions.

Table 1 estimates how much moisture is available to a vegetable crop between saturation and moderately dry or dry conditions for different soil textures.  This table can be a guide for how much water should be applied to re-saturate the soil.  For example, applying 0.42 inches per foot of rooting depth will bring a moderately dry silty clay soil back to saturation.  Applying more than this amount of water will likely over-saturate the root zone.

Table 1. Estimated plant-available moisture for different textured soils.      

Also, estimating the cumulative crop ET since the last irrigation can guide how long to irrigate. Reference ET values between south Salinas and Soledad during this hot spell have been as high as 0.27 inches per day.  If the crop has a full canopy, 0.25 to 0.3 inches for each day since the last irrigation would be a good rule of thumb for how much water to apply as long as the total does not exceed the water holding capacity of the soil.   

Lastly, one needs to convert the amount of water to apply to an irrigation run-time.  To make this calculation one needs to know the application rate of the irrigation system. For impact sprinklers, the application rate can be estimated using Tables 2-4.  Note that pressure and nozzle size have a significant effect on application rate.  For drip, the irrigation time will depend on the tape discharge rate and pressure, as well as the spacing of drip lines.  Assuming that the drip system is operated at the pressure recommended by the manufacturer (usually 8 to 10 psi) one can use Table 5 to approximate the application rate.  For example, for one drip line of medium flow tape (0.45 gpm/100 ft) on 40- inch wide beds the application rate of the drip system is 0.13 inches per hour. If there are several drip lines per bed then multiply the application rate in the table by the number of drip lines.

The appropriate run-time can be estimated by dividing the amount of water to apply by the application rate of the irrigation system.  For example, to apply 0.6 inches of water to a field with drip using medium flow tape the water would need to run for 4.6 hours:

Hours to operate the irrigation system = 0.6 inches of water/0.13 inches per hour = 4.6 hours 

Summary

Irrigating the right amount of time to bring the soil back to saturation will maximize crop ET during these hot days, and hopefully prevent any heat damage to crops.  Also, consider visiting the CropManage website (cropmanage.ucanr.edu) for further guidance on scheduling irrigations. This online tool can assist growers in quickly estimating how much water to apply to meet crop water needs.  

Table 2.  Sprinkler application rate for varying pressures and nozzle diameters for a solid set spacing of 30 × 30 feet (Rainbird 20JH).

Table 3.  Sprinkler application rate for varying pressures and nozzle diameters for a solid set spacing of 30 × 33.3 feet (Rainbird 20JH).

Table 4.  Sprinkler application rate for varying pressures and nozzle diameters for a solid set spacing of 30 × 40 feet (Rainbird 20JH).

Table 5.  Drip application rates for varying bed widths and tape flow rates estimated for 1 drip line per bed.  Multiply the rate in the table by the number of drip lines per bed to determine the actual application rate.  (For 3 drip lines on an 80-inch bed multiply by 3)