Choose Tomato Varieties With The Best Disease-Resistance Packages

Editor’s Note: Zitter and Mutschler-Chu are both Cornell University researchers. Zitter is in the department of plant pathology and plant-microbe biology and Mutschler-Chu is in the department of plant breeding and genetics; [email protected].

By Tom Zitter and Martha A. Mutschler-Chu

Selection the best tomato cultivars to grow involves more than choosing the best color, shape, and size. Purchasing varieties with the most complete disease resistance package should now be a consideration for reducing losses to diseases, not to mention the added cost and time for any fungicide sprays.

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Introducing Tolerant Cultivars
During the past few years, new cultivars with tolerance for early blight, (EB), and resistance to late blight (LB) and Septoria leaf spot (SLS) have been introduced.

These three diseases are responsible for most tissue defoliation, and two (EB and LB) result in high fruit losses. This article examines what genetic controls are possible for these three diseases, a summary of what to expect with each genetic offering among commercially available cultivars (see Disease Resistant Hybrid chart), and how best to use them, including the newest triple resistant cultivar, Iron Lady.

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Disease-Resistant-Tomato_Chart

Early Blight Control
The current genetic control for early blight is tolerance. It is questionable how much protection is provided when the tolerance is only in the heterozygous condition. For example, when Mountain Merit (see chart on page 50), which is heterozygous for EB tolerance, was tested in research trials, it showed less infection than a fully susceptible cultivar, but will still have considerable defoliation and may require some fungicide applications.

The level of tolerance is beneficial in plants homozygous for it, but this tolerance does not provide complete control. The stems are kept clean of all but small lesions, preventing early plant collapse. Leaves can still sustain substantial lesions and disease can develop.

Fruit lesions are rarely a problem. It is good to supplement this tolerance with core horticultural practices:

  • rotation out of tomato/potato crops for at least two seasons;
  • no solanaceous weeds in the field during this period, and
  • an adequate nitrogen fertilization program.

If minimal fungicides are required, select those with low environmental impact quotient (EIQ) values like Quadris Top (Syngenta) or the protectants chlorothalonil or mancozeb, and make the first application after fruit set occurs. For organic production, there are several copper fungicides from which to choose.

Genes Resistant To Late Blight
For late blight, three different genes have resistance in tomato.

Ph1: This gene is present in the old cultivar New Yorker. Ph1 is only known to control earlier genotypes of the late blight pathogen, but not to any of the current genotypes like US22 or US23. This gene is therefore not used in modern cultivars.

Ph2: This one was found in the old cultivar West Virginia 63. Ph2 is effective against only some genotypes of the pathogen. Ph2 is not very effective as a stand-alone genetic control. This gene is also found as solo homozygous resistance in the cultivar Legend, and should not be relied upon for LB control.

Ph3: Ph3 was found in a wild cherry tomato, and transferred into a number of tomato lines. This gene is almost dominant. Mild disease can still be present on hybrids heterozygous for Ph3, but hybrids homozygous for this resistance have virtually complete resistance against almost all genotypes including US22 and US23.

The best control of late blight occurs when the hybrid is homozygous for both Ph2 and Ph3. To date, no genotype of late blight pathogen has been found to cause significant disease on such hybrids.

Dealing With Septoria Leaf Spot

Iron Lady is a new variety from High Mowing Organic Seeds. Photo credit: High Mowing Organic Seeds

Iron Lady is a new variety from High Mowing Organic Seeds.
Photo credit: High Mowing Organic Seeds

This appears to be a nearly dominant single-gene resistance. We are still trying to determine if plants homozygous for the SLS gene have stronger resistance than heterozygous plants. SLS resistant plants develop initial lesions that stay small, but SLS resistance strongly impedes pathogen reproduction, which suppresses epidemic development of this disease.

Maximal SLS control is obtained by minimizing initial sources of inoculum, so grow resistant hybrids up-wind/separated from susceptible hybrids, in addition to using good core horticultural practices (mentioned under early blight).

If seasonal rains occur during June, July, and August and a three-year rotation out of tomatoes was not adhered to, then expect that one or two fungicide sprays will be required. The spread of SLS can explode dramatically, especially with splashing water, either natural or via overhead irrigation. Fungicide programs are the same as described under early blight.

Development of resistant lines and hybrids using conventional breeding practices has been a goal of a number of breeding programs over the past decade. New hybrids have and are being released; Iron Lady has triple resistance as discussed. The current cultivars combining resistance to two or three of these diseases are summarized in the chart below.

How To Use Available Cultivars
Disease control recommendations are predicated on the need to follow certain core horticultural practices. Crop rotation out of tomato for two to three seasons is one of the most important in terms of reducing soilborne inoculum for either EB or SLS. This is not possible in all situations, thus the influence of these two diseases needs to be considered.

Also, if you compost and have included tomato debris from the previous season, you may be introducing EB and SLS, even if rotation is followed. If you interplant these cultivars, EB or SLS with fully susceptible tomato cultivars, you can expect to have some inoculum spread into these tolerant or resistant cultivars.

In general, growers who follow TOMCAST, a weather-based disease predicting system, will be able to increase the disease severity value (DSV) index to 25 or higher DSV for their weekly calculations as to when sprays are necessary. If SLS is a threat on your farm, then a closer adherence to the spray schedule is necessary since SLS can explode during a summer with frequent showers. Still, a maximum of two sprays may be sufficient.

In the case of EB, TOMCAST can be followed using the same criterion. A single spray will be necessary. Make sure that no strobilurin fungicides are used alone because of widespread resistance for EB among all Group 11 fungicides. These fungicides can be used if mixed with another effective fungicide, (ie. Quadris Top, Quadris Opti, Syngenta).

In general, LB is not an issue if using the cultivars shown in the chart and due attention is paid.

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[…] Choose Tomato Varieties With The Best Disease-Resistance Packages – Selection the best tomato cultivars to grow involves more than choosing the best color, shape, and size. Purchasing varieties with the most complete disease resistance package should now be a consideration for reducing losses to … […]

Avatar for DitkasHair DitkasHair says:

Where’s the Disease Resistance Chart? I mean, WTF, my tomatoes started with this s**t a couple of years ago, I’ve only got a small amount of space to plant so it’s not like ‘crop rotation’ is going to be possible or of any consequence … shoot it even hits the tomato plant that’s in a big pot at the other side of the house, it’s friggin crazy!!! It’s either Fusarium wilt or Verticillium wilt, and hell, maybe even Septoria leaf spot …

Avatar for Edwin Grosvenor Edwin Grosvenor says:

Thank you for this excellent, every helpful article. But your chart “Disease Resistant Hybrid Tomatoes” which is a critical part of the essay is so low in resolution that is unreadable.

Avatar for Paul Nye Paul Nye says:

An answer to the comment by Edwin Grosvenor. If you make a screen shot of the chart and then open the screen shot and increase the size it will be larger and in focus. I use this method for many visual charts. I hope this helps.

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