Organic Treatments for Combating Common Spinach Diseases Brian DeRubes and Antoinette Pereira

Environmental Studies - Science for Sustainable Communities Fall 2017

Introduction

Spinach growers in the Salinas Valley are impacted by two common spinach diseases that

negatively affect the marketability of their crops. Hyaloperonospora brassicae and

Pseudomonas spp, commonly known as downy mildew and leaf spot, are two common diseases

found in the Salinas Valley that this study wishes to address. Both diseases are airborne, making

contamination almost unpreventable causing majority of fields to be infected. Conventional

treatments for these diseases involve the application of different commercial fungicides that do a

relatively good job of killing the bacteria/fungus responsible for the diseases. However, the

organic Ag industry is on the rise and face different challenges when it comes to treating leaf

spot and downy mildew. With chemical use highly restricted organic growers must utilize

alternative disease treatments.

Natural ways of managing downy mildew and leaf spot must be studied to aid organic

growers in the fight against downy mildew and leaf spot, to produce the highest yields of

marketable spinach. Information on organic treatments for leaf spot and downy mildew will

better equip the organic industry to compete with conventional Ag and potentially phase out the

use of chemical treatments if organic methods prove effaceable.

To elaborate upon the body of research regarding organic treatments for downy mildew

and leaf spot we partnered with the Agricultural Land Based Training Association (ALBA),

located in the Salinas Valley. Our goal is to refine existing methods for mitigating these two

common diseases by studying four organic treatment methods and their efficacy. This research

aligns with ALBA’s over-arching mission of advancing the economic viability, social equity, and

ecological land management techniques amongst limited-resource aspiring farmers. Their 100

acre training farm/facility, dedicated to teaching and advancing organic farming techniques,

made ALBA the perfect location to conduct further research on downy mildew and leaf spot.

Under the supervision of Dr. Robina Bhatti, who manages a very productive 1 acre field

at ALBA and teaches an agroecological course at CSU Monterey Bay, we developed a field

experiment to test the most effective organic methods (and their combinations) for mitigating the

spread and severity of downy mildew and leaf spot. We decided focused our research on

spinach, though the diseases affect other leafy greens, because spinach is very susceptible to both

diseases, has a quick seed to harvest time (about 4-5 weeks), and is very commonly grown in the

Salinas Valley.

Background

A study conducted by the University of Arkansas found Hyaloperonospora brassicae

(downy mildew) as the most common disease in spinach cultivation due to its airborne spread. It

noted that within the past 2 years the severity of the disease has risen, due to increasing densities

at which spinach is planted, since it one of the most commonly sold greens; new strains have

thus multiplied rapidly over the years making the disease increasingly harder to treat. The study

concluded that, over the years, downy mildew has had negative impacts on the price of spinach

and even causing some growers to lose their fields.

Figure 1A: spinach leaf infected with downy mildew. Figure 1B: spinach leaves infected with leaf spot.

In 1997 a new disease of spinach was identified in the Salinas Valley known as

Pseudomonas spp (leaf spot). The disease in its early stages does not affect the overall quality of

the crop and resembles water spots, however, if left untreated these spots merge together and

form large dead purple areas on the foliage. Untreated leaf spot can put entire crops at risk

causing growers to lose large percentages of their marketable greens. Like downy mildew leaf

spot is spread through the air making it another common disease spinach growers have to deal

with.

The principles of combating downy mildew and leaf spot organically, without chemical

fungicides, is to first utilize a fabric barrier (or row cover) to protect crops against airborne

diseases. Row covers are normally implemented a week or two after greens have sprouted. The

second principle for fighting the common diseases in question is to create very healthy and

robust plants, capable of fighting disease on their own (similar to the human body’s immune

system). This is accomplished through the use of organic fertilizers, and other techniques, to

give plants everything they need to thrive. Soil amendment can be implemented anywhere from

the beginning to end of the planting season and the fertilizers recommended to us by our

community partner, and the ones used for this experiment, were worm castings and liquid kelp,

because of their high concentrations of macro/micro nutrients. We also experimented with

intercropping snap pea with the spinach because peas provide nitrogen to the soil which we

hoped would be another amendment for growing stronger more resilient plants.

Approaches

Dr. Robina Bhatti, our community partner, wanted to know how she could best mitigate

downy mildew and leaf spot in her organic field. Listed below are the different approaches that

could be used to treat these diseases.

Approach One

Treat crops with a combination of lightweight row covers, intercropping, liquid kelp, and

worm castings to build healthy/heartier plants more resistant to diseases.

Approach Two

Treat spinach with chemical fungicides to kill bacteria which produce diseases.

Approach Three

Use genetic engineering to create genotypes of spinach resistant to diseases.

Approach Four

Deploy spore traps, within the field, to help sequester the spread of airborne diseases

before they reach crops.

We decided to incorporate Approach One into our experimental design because growing

on an organic farm greatly restricts what soil amendments/agrichemicals we could use. All

amendments had to be pre-approved by ALBA before use to align with their organic

certification. This approach also is the most cost effective, since worm castings and liquid kelp

are so concentrated and only need to be used in sparing amounts. Lastly, Approach One is

completely natural has no environmental consequences unlike Approaches Two and Three. By

incorporating Approach One into our experiment we hoped to both increase the productivity and

quality of our community partner’s spinach harvests.

Experimental Design

To design our experiment, we had to first prep four parallel rows, about seventy feet in

length, by hoeing weeds, shaping beds, and laying drip tape. We then dedicated rows one and

four (the outermost) as our treatment/experimental rows that received a combination of all four

disease treatments: fabric row covers, worm castings, liquid kelp, and pea plant intercropping.

Rows two and three (the innermost) served as controls and only received row covers and liquid

kelp. We did not feel the need to alternate between control and treatment rows since both downy

mildew and leaf spot are airborne and so the location of rows would not affect disease spread.

The organization of our data (how we collected our measurements) was designed alongside Dr.

Steven Kim, faculty member of CSUMB’s Department of Math and Statistics, to ensure the data

collected would be reliable and account for any confounding variables and potential sampling

errors.

We conducted a random sample of our four spinach rows by laying transects across each

row and marking every fourth foot with a flag. There was a total of 17 flags per row totaling to

68 flags. Each flag was given an ID number (1-68) and sampled one foot squared around every

ID flag. One week after seeds had sprouted all rows were covered (first treatment). Two weeks

after planting we conducted a pre-sample recording density of spinach affected by leaf spot and

downy mildew. After the pre-sample we applied worm castings and liquid kelp to the respective

control and treatment rows.  Next, we sampled our one-foot squared areas around each of the 68

data points by measuring/counting every individual spinach plant found within the one foot area

of our ID flag. We then counted how many of those individuals were affected by leaf spot,

downy mildew, and pest damage through visual observation. If any part of the plant showed any

symptom the whole plant was counted. This sampling procedure was carried out a week for a

total of three weeks. Lastly, when harvest time came, each square foot sample was cut and

weighed to determine the overall productivity between control and treatment rows by weight in

grams. Our specific goal throughout our study was to determine how effective lightweight fabric

row covers, liquid kelp, worm castings, and intercropping were in mitigating leaf spot and

downy mildew as well their relationship to overall productivity.

Results

In

total, we

sampled

sixty-eight

square feet

of our entire spinach crop and recorded data on the following four variable: spinach density,

number of individuals affected by downy mildew, number of individuals affected by leaf spot,

and number of individuals affected by pest damage. These data sets were collected once a week

for three weeks to determine change over time of pest damage and disease spread/severity.

Week one was our pre survey (before application of liquid kelp and worm castings), followed by

our post surveys in weeks two and three (after application of liquid kelp and worm castings).

The results from these three weeks of data collection were organized into our Master Excel Sheet

and again we collaborated with Dr. Steven Kim on the graphical and statistical analysis of this

field data— below is our analysis.

Spinach Density (number of individual plants)

Figure 2A: sprouted spinach in treatment

row one.

Figure 2B: transect along which we

sampled every, one foot squared, every

four feet, along each row.

Over the three week period, the treatment group showed a decrease in spinach density by an

average of 1.1 individuals, whereas the control group showed almost no change.

Total Weight (grams)

Figure 4A: combined weight of spinach between control and treatment rows.

Figure 3A (above): change in spinach density over time for control and treatment rows.

The treatment group was slightly less productive in terms of grams harvested than the

control group.  In the treatment group, the average weight in grams per square foot was 18.2. In

the control group, the average weight in grams per square foot was 21.7. The p-value for

comparing the control group and treatment group was 0.0652. When comparing all four rows

separately there is a greater distinction, in terms of weight, between individual rows: row one

(T1) averaged 23.2 grams/square foot, row two (C1) 20.1 grams/square foot, row three (C2) 24.3

grams/square foot, and row four (T2) 13.2 grams/square foot.

Figure 4B: individual weight of spinach per rows one through four after harvest.

Figure 4C: researchers weighing freshly harvested spinach in the field.

Leaf Spot (number of individual plants affected)

Both the control and treatment groups showed a decrease in proportion of spinach affected by

leaf spot. It appears that the treatment group is less effective than the control group in preventing

this disease. In the treatment group, the proportion affected decreased by an average of 0.143.

In the control group, the proportion affected decreased by an average of 0.344 (p-value of

0.000395).

Downy Mildew (Yellow)

Both treatment and control groups showed an increase in proportion of plants affected by

downy mildew over the course of the survey period.  However, it appears the treatment group is

Figure 5A: change in proportion of individuals affected by leaf spot over time.

Figure 6A: change in proportion of individuals affected by downy mildew over time.

more effective in slowing down the spread of downy mildew than the control group. In the

treatment group, the proportion affected increased by 0.187 on average. In the control group, the

proportion affected increased by an average of 0.362 (p-value = 0.000151).

Pest Damage

Both groups showed an increased proportion of spinach affected by pests. There is not

enough statistical evidence to prove which group was more effective in mitigating pest damage.

In the treatment group the proportion affected increased by 0.236. In the control group the

proportion affected increased by 0.261 (p-value = 0.566).

Evaluation

From the above analysis we can conclude the following about our four disease treatment

methods. Worm castings is not very impactful, when used in conjunction with liquid kelp, for

reducing the spread/severity of leaf spot. However, worm castings did prove impactful in

conjunction with liquid kelp for mitigating the spread/severity of downy mildew. Since downy

mildew and leaf spot often occur together in the Salinas Valley, it would be wise for growers to

utilize both fertilizer amendments to prevent the spread/severity of both diseases. Row covers

were critical in mitigating downy mildew and leaf spot and had we not covered our crops early

we would not have had a successful harvest. Lastly, it is difficult to tell whether intercropping

had a positive impact, since there is no pre- and post-measurement for this treatment because

peas had to be planted the same time as spinach.

Figure 7A: change in proportion of individuals affected by pest damage over time.

Intercropping pea plants was a confounding variable and should have excluded from this

experiment. When we weighed our sample plots we excluded peas on the basis that we were

only interested in the outcome of spinach, however, the weight of the peas should have been

included (as total biomass) since by intercropping peas we lost the opportunity of planting more

spinach. This could be one of the reasons why our treatment rows had lower average weights

than the control rows.

The limiting factors we encountered when conducting this research was time and labor.

We only had 5 weeks to plant, measure, and harvest spinach before the rows we planted had to

be tilled. We would have liked to run the experiment at least a couple weeks longer to let the

spinach fully mature and gather more data on disease rates and pest damage over time. Labor

was also a limiting factor because farming is hard work, without the experimental design and

data collection requirements. Due to time and labor restraints we had to plant treatment row four

five days after we planted the other three. This delay would not affect disease rates; however, it

did skew our overall weight measurements since the treatment row was less mature. This would

account for the lower average weight of spinach in our combined treatment rows (Figure 4A).

Recommendations

For future studies we recommend focusing on as few disease treatments as possible to

avoid confounding influences. Focusing on one treatment at a time is the best way to determine

treatment efficacy. We tried to test too many things at once attempting to answer to many

questions in one growing season. We should have excluded intercropping and stuck with the

addition of worm castings as the only other additional treatment. It is crucial to be as focused as

possible when it comes to treatments because combinations can be confounding making the

efficacy of each treatment vague.

We also recommend a larger team for conducting this type of experiment because the

five weeks spent in the field was very physically demanding. Also, working with more people

on a larger sample size, could yield greater data sets that would be more conclusive over longer

sample times. Lastly, planting should begin as soon as possible to avoid any unforeseeable time

restraints that may arise since farming is such a dynamic trade.

Conclusion

Overall, we grew a very productive amount of spinach without the use of chemical

fungicides. We provided valuable insights to our community partner, and hopefully all organic

spinach growers of the Salinas Valley, on how to best treat downy mildew and leaf spot.

Growers should first cover spinach crops as early as possible to reduce the initial spread of

diseases. Next, they should invest in liquid kelp fertilizer to mitigate downy mildew and leaf

spot and begin fertilizing as soon as possible. The liquid kelp also gave our spinach a rich deep

green color very sought after in the organic market. Lastly, if plants are more infected with

downy mildew growers should invest in worm castings as another soil amendment to combat

against this. Since both diseases are so common, organic spinach farmers should consider the

combination of both liquid kelp, worm castings, and over course row covers to fortify their crops

as much as possible against disease.

Working with our community partner at ALBA gave us a much better understanding of

what it takes be an organic farmer. This was our first time growing anything on this large a scale

and we were very proud of how productive our spinach was. Organic farming is a lot of work,

we spent many hours in the sun backs bent pulling weed after weed. However, through all the

labor we were connected to the Earth and growing food seemed like a natural thing, almost

intuitive. There is so much to learn of course, but organic culture will one day dominate the

industry since it is the only long term, regenerative, and sustainable option available to human

kind.

References

Agriculture & Land-Based Training Association. ALBA Farmers. 2017 [accessed 2017 Nov

10]. http://www.albafarmers.org/

Alexander LM. The Battle Against Downy Mildew In Spinach. Growing Produce. 2015 Dec 2

[accessed 2017 Oct 10]. http://www.growingproduce.com/vegetables/the-battle-against-

downy-mildew-in-spinach/

Dicklow B, Wick R, Hazzard R. Cercospora Leaf Spot of Swiss Chard, Beets, and Spinach.

Center for Agriculture, Food and the Environment. 2013 Jan 17 [accessed 2017 Oct 10].

https://ag.umass.edu/vegetable/fact-sheets/cercospora-leaf-spot-of-swiss-chard-beets-

spinach

Facts, Figures & FAQs. Monterey County Farm Bureau - Facts, Figures & FAQs. 2017 [accessed

2017 Nov 10]. http://montereycfb.com/index.php?page=facts-figures-faqs

Klosterman SJ. Development of a Pathogen Detection Early Warning System for Spinach Downy

Mildew. USDA: Research, Education and Economics Information System. [accessed 2017

Nov 16]. https://portal.nifa.usda.gov/web/crisprojectpages/0424554-development-of-a-

pathogen-detection-early-warning-system-for-spinach-downy-mildew.htm

Koike ST, Henderson DM, Butler EE. Leaf Spot Disease of Spinach in California Caused by

Stemphylium botryosum. 2001 Apr [accessed 2017 Oct 10]. https://aggie-

horticulture.tamu.edu/extension/newsletters/vpmnews/apr01/art2apr.html

LeStrange M, Koike ST. How to Manage Pests. UC IPM Online. 2009 Dec [accessed 2017 Oct

10]. http://ipm.ucanr.edu/PMG/r732100711.html

Mou B, Koike ST, duToit LJ. Screening for Resistance to Leaf Spot Diseases of Spinach.

HortScience. 2008;43(6):1706–1710.