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2021 Considerations: Tomato Time

Authors: Natalie Hoidal and Marissa Schuh, Extension Educators, University of Minnesota Extension

Our winter / spring vegetable series focused on helping you anticipate and prepare for the growing season’s potential problems continues. Let’s talk tomatoes! Two important tomato management strategies to prepare for before the season begins are your fertilizer program, and bacterial disease management.

Fertilizer programs for tomatoes

Of all of the vegetables, tomatoes tend to get the most attention when it comes to fertility programs. With more and more farmers growing tomatoes in tunnels, (there were over 2000 high tunnel tomato growers in the 2019 MN census of horticulture!), we’re seeing increasing interest in utilizing foliar and sap testing, and applying specialized fertigation treatments. There are a few key things to think about when developing your management plan. The following are universal concerns for high tunnels.

  • Make sure you’re testing your soil very regularly, at least annually in your high tunnel. The pH in tunnels tends to increase over time, and soluble salts like calcium can quickly accumulate. These changes can impact the uptake nutrients, and so it’s critical to keep a very close eye on them. You should take a basic series soil test each year in your tunnel, as well as a soluble salt analysis. It’s not a bad idea to conduct extra tests like calcium, magnesium, zinc, iron, boron, and copper, at least every couple of years.
  • Keep an eye on your pH. The ideal soil pH for high tunnel tomatoes is 6.0 - 6.8. Sulfur can be applied in the fall to help moderate soil pH levels if they are increasing.
  • Test your irrigation water. In covered systems that are not receiving regular rainfall, the pH of the soil tends to increase, and can do so rapidly. The pH of your irrigation water should be between 6.2 and 6.5 for tomatoes, and can adjusted using citric acid in organic systems, or a wide variety of other acids such as sulfuric, phosphoric, or nitric acid in non-organic systems. Alck Calc is a helpful, free tool to help you adjust your irrigation water properly. If you have a high pH but did not apply sulfur in the fall, this is a way to bring the pH down in the short term for the coming season. 
  • Phosphorus tends to be quite high in tunnels, and in general on organic farms. Due to higher inputs of manure and compost in tunnels, we often see very high phosphorus levels in the soil. This can be problematic from an environmental stewardship standpoint. If you’re seeing high levels of phosphorus, focus on adding low phosphorus, high nitrogen fertilizers, and using cover crops rather than composts to build organic matter. 
  • Avoid the build-up of soluble salts. In general, try to avoid fertilizers with a high calcium content to avoid accumulation of salts, and try to leave the hoop house open over the winter in years when you replace the plastic to allow the accumulation of rain and snow in the soil.
  • Invest in irrigation management tools. Consistent irrigation is critical for high quality fruit, and for preventing issues like blossom end rot. Rather than following a calendar schedule, consider investing in moisture sensors and irrigating according to soil moisture levels. More info about irrigation management in vegetables.
Tomatoes growing in a high tunnel. Photo NH

Foliar and sap testing: Tomatoes require relatively little nitrogen during the first couple months of development, but as they begin to flower and produce fruit, their nitrogen demand increases substantially. However, nitrogen is quite mobile in the soil, and tends to leach. As such, many growers, especially in tunnels, use fertigation (fertilizer through the drip line) to supply nitrogen as plants mature. Fertigation also allows you to provide your plants with a relatively quick source of fertilizer if anything is lacking. There are many water-soluble sources of nitrogen for conventional growers; organic growers can find a comparison of soluble N sources from Cornell here.

For growers using fertigation, foliar sampling is recommended at least a few times each year to help you fine tune your fertilizer inputs, particularly for N and K. The Nutrient Management Guide for Commercial Fruit and Vegetable Growers in Minnesota has target nutrient ranges for foliar tests at different stages of development; it’s recommended that you conduct a foliar test at the onset of blooming, early in fruitset, and mid-way through fruit production. Compare your results to the ideal range in the guide, and adjust accordingly. Each foliar sample should include 12-15 full leaves (a tomato leaf consists of all of the leaflets along a petiole, just just an individual leaflet), and the leaves you select for your sample should be the most recently matured leaves on each plant. *Keep in mind that foliar sampling should not stand on its own; it is supplemental to soil testing*. You can read more about foliar sampling here.

A few Minnesota growers have ventured beyond foliar sampling to sap testing, an even more fine-tuned approach to nutrient management. The argument goes: foliar testing gives you a snapshot of what the plant has already taken up, whereas sap testing gives you a preview of what is coming. Plant sap analysis can show you the concentration of nutrients being actively taken up by the plant. This method is only useful if done on a weekly basis, and most of the Minnesota growers currently using it ship their samples to the Netherlands. While this can be a valuable tool, we recommend sticking to the basics first until you have perfected them, and then considering this next step. This method is fairly expensive and time consuming, and is really only worth it if you: 1. have the time to make weekly adjustments, and 2. will benefit greatly from a minor yield boost. As such, this method is quite valuable for growers with multiple tunnels for whom tomatoes make up a substantial portion of the farm’s income, but for others it may be more work than it’s worth. 

Depiction of entire leaf, including leaflets and petiole. Source: MN Nutrient Management Guide

Bacterial Disease Management

The last few years have seen an uptick in the occurrence of three bacterial diseases in tomatoes: bacterial spot, bacterial speck, and bacterial canker. Control of all of these diseases starts during transplant production. These diseases are known to be seedborne, and the trays, tables, and tools used in the greenhouse provide places for the disease to survive from year to year. Like all bacteria, water is key in moving the bacteria from plant to plant, and wounds provide ways for pathogens to enter the transplants.

Greenhouse Sanitation

Good greenhouse sanitation is the basis of bacterial disease control, especially if you struggled with one of these diseases last year. Try not to reuse transplant trays, and give tables and tools a good cleaning and sanitizing before the season starts. Wood tables aren’t sanitziable, but a good cleaning followed by adequate drying time will help reduce places the bacteria can be harbored.

Starting off Right

Some seed suppliers do some level of treating or testing on seeds, while others do not. It is worth a conversation with your seed supplier to learn about what they do.

Seed treatment on your own can be an option, though it may void some guarantees from the seller. Doing seed treatment on your own is an exact science, and must be done in very particular ways to kill pathogens while not reducing germination. More information on this practice is available from Ohio State University and Michigan State University.

Scouting Transplant and Roguing

Scout transplants carefully. Symptoms on transplants can look different from the bacterial disease symptoms you may be familiar with in the field. Look for:

  • Dark black or brown spots with yellow halo
  • Tan spots
  • Blotchy areas on steams and/or leaves

Bacterial Spot lesions on the underside of the leaves of a tomato transplant. Photo: Cheryl Truman, University of Guelph.

What to do When Diseased Plants are Found

The most important aspect of bacterial disease control is roguing diseased transplants. Research has shown these diseases can move quickly throughout a greenhouse, and that tomato transplants can have bacterial diseases before they show symptoms. This means that roguing should include both plants with disease symptoms and their neighbors.

Research in Florida on bacterial spot has shown that overhead watering allows bacterial spot to move throughout transplant production trays. In the study, one tray of tomato transplants was inoculated with bacterial spot, and placed in a transplant production greenhouse. Within 5 days, the bacteria has moved anywhere from 4 to 11 inches beyond the initial point of infection. After 12 days, the bacteria could be found five to ten feet away. The plants the bacteria were detected on didn’t necessarily show any symptoms of infection yet, with a 5-7 day lag between infection and symptoms appearing. Research in Michigan and the Netherlands has shown bacterial canker can move through transplant trays as well.

Greenhouse conditions (warm, humid, moist) are universally conducive to the movement of bacterial diseases. Symptoms on one plant this week could turn into a trays of infected tomato plants by the time transplanting rolls around (and remember, plants aren’t going to be “cured” in the field).

Bacterial canker causing the edges of leaves to die and wilt. Photo: Zachariah Hansen, University of Tennessee.

Diseased plants should not be composted, instead throw the tray away to get a potential source of future disease off the farm.

Treatment Options

If you plan to use sprays, sprays used preventatively in the greenhouse will do more for your plants than bacterial-specific sprays once they are in the field. Copper works on some strains of some bacterial tomato diseases. Products with the active ingredient streptomycin can also be used while transplants are in the greenhouse. For full information on chemical controls, see the Midwest Vegetable Production Guide.

Some additional tomato resources

  • It’s always helpful to learn about the practices of other growers. Each year at the Organic Vegetable Production Conference in Wisconsin, three different farmers share a detailed review of their practices for a specific crop. These spreadsheets are available online. You can read the profiles of 3 tomato growers and their practices from 2015, and 3 profiles from 2018.
  • Last summer we hosted a discussion about high tunnel fertility on the Great Lakes Vegetable Producers Network with guests Juson Reed from Cornell and David Van Eeckhout from The Good Acre. Listen here: (podcast episode titled Hoophouse Nutrient Management)

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