Note to readers: I wrote this to add to information in the SF Chronicle Article that will appear in the print version of the paper on March 4, 2012. The article will not be availabe on sfgate.com until Monday, March 5th, so if you are here on Saturday or Sunday, and want to find out what is in the original Chronicle article, check back on sfgate.com on or after Monday.
Are you losing tomato plants to tomato late blight? This disease has been killing tomato and potato plants in the Bay Area for several years, and I have been trying to find tomato varieties that will resist it. I reported last year’s trial in the SF Chronicle and on sfgate.com. (If you haven't seen that articlet yet, you can find it at http://www.sfgate.com/columns/goldengategardener/archive/ .) In that article, I promised more details here, in my blog, so here they are. (Note: If you are an expert reading this post, and have corrections or more information to offer, please comment or send me an email through my website at pampeirce.com.)
This is a long post, so I will tell you what is in it here at the beginning. The first section explains what late blight is, with photos of symptoms. The second section is a brief explanation of ways to try to reduce late blight damage in your garden. Section 3 tells you how to set up a tomato variety trial yourself, with photos to show you how we did it. The fourth section includes some details about genetic resistance to late blight among available tomato varieties, with photos of the fruits of the ones that succeeded in the 2011 trial. The fifth section tells you how to send samples of your blight to the website www.usablight.org in order to identify the clonal line(s) of the blight present in your garden. Then, finally, at the end of this post, is the list of varieties that also appears in the sfgate.com column, showing you which I trialed in 2009 and 2011, plus a few new varieties that I haven't yet trialed.
What is tomato late blight?
Tomato late blight is caused by the oomycete Phytophthora infestans. (Oomycetes are organisms that used to be called “water molds.”) The disease-causing organism is in the same genus as the one that causes sudden oak death, Phytophthora ramorum, though the two diseases infect different groups of plants.
“Tomato late blight” is a common name. Because the same organism also infects potato plants, it is sometimes called “potato late blight” or even “tomato and potato late blight.” It can also infect petunias, which are in the same plant family; though I have never heard it called “petunia late blight.”
The words “late blight” are found in the common names of other plant diseases that are caused by different disease organisms. For example there is “celery late blight,” which is caused by the fungus Septoria apii. (Just one more example of why it is important to know the scientific name of a living creature, even if you commonly use the common name.)
If a plant is sucseptible, lesions start to appear all over and spread, killing leaflets, whole leaves, and stems. If fruit has formed, it soon has greasy, brown "shoulders." If it ripens at all it wil have poor flavor.
The tomato disease that causes our grief is known as a “late” blight because the symptoms typically show up only when the plant is mature, the fruit has set, maybe even some has begun to ripen. (By the time it shows up, we have usually had a couple of months to tend the plants and eagerly monitor the progress of setting fruit, so our disappointment is even deeper than had the disease struck early, before we had developing fruits to mourn.) The late onset happens because the mild temperature and moist air that encourages the spores to grow is likely to occur later in the summer, and also because as infections start to occur, there are more spores to blow about in the air and spread the disease.
It’s important to know that the spores are not in the soil, but are blown in the wind from plant to plant, from as far as several miles away. They also are not carried in seed.
It seems that we are fortunate, if that is possible, considering the damage this disease causes if it strikes, because so far, we have only asexual zoospores in our part of the world. These zoospores do not survive in soil, only in living or recently living susceptible plants. Because of this, we can get most of the spores out of our gardens by making sure we have no living plants that can get the blight in our gardens for four months in the winter. In our community garden, we remove all living and dead material from tomato, potato, petunia, and wild solanums (nightshades) from the garden in November through March. (If you do this, you also have to watch for and remove volunteer potatoes that can come up in winter and sometimes get late blight during a warm spell in winter and early spring.) While removing susceptible plants in winter can reduce the number of spores around in summer, it can’t, of course prevent new spores from blowing in, so you still may find the disease in your garden, but at least it isn’t a certainty.
(If the form of the organism necessary for sexual reproduction were to find its way here, we could have sexually produced oospores here that would survive in soil and live longer than the asexual zoospores. Fortunately, there is no sign of this having happened or immediate expectation that it will.)
Protecting Tomato Plants from Late Blight
To protect plants from this disease, gardeners try to keep leaves dry, shelter plants from spore-bearing breezes, or spray them with a protective substance. In the Northwest, tomatoes are often grown under row cover (polyester fabric that lets most light through) or plastic hoop houses to keep rain off. In the Bay Area, we aren’t likely to have rain in summer. Our high humidity can provide enough moisture for the spores to germinate, but you may as well avoid wetting the leaves when you water, and you can try putting plants under cover if you like.
Fungicides based on copper compounds and the fungicide Serenade are both registered to prevent tomato late blight and are permitted to organic farmers. Either kind of fungicide needs to be sprayed starting from the time you set out your transplants, and continued all season following label instructions, which are to spray more or less weekly. If they are used heavily, copper compounds can increase the copper content of soil so much that plants grow less well, so perhaps they aren’t the best idea. I did try spraying a copper fungicide one year. I found it a lot of work to spray my small planting so many times—and the plants still got late blight.
I haven’t tried Serenade for late blight yet. It is based on the harmless soil bacterium Bacillus subtilis, and it is worth a try if you don’t mind the spraying. (I was able to prevent onion downy mildew from overtaking my perennial “walking onions” by spraying them with Serenade, so I do know that it works against that disease--another one listed on its label.)
Looking for Resistance to Tomato Late Blight
In addition to the winter clean up, which is basic to prevention, I have been trialing varieties that might stand up to this disease. I would very much like to grow my favorites, such as ‘Cherokee Purple’ or ‘Old German’ heirloom varieties, but they get late blight. I am looking for varieties that are resistant to late blight because I’d rather have some tomatoes for sure than spend all my time spraying or covering my plants and likely still losing them to the disease.
The history of my tomato variety trials is in my sfgate article, (http://www.sfgate.com/columns/goldengategardener/archive/ ) along with a summary of how to do such a trial. Here are more detailed instructions for those who might want to carry out a trial.
To do a trial, you need one or more experimental varieties (ones you have some reason to believe will resist the blight) and one control variety (one you know will get the blight). The control variety is there to prove that the disease was present in your garden, because if it wasn’t, you didn’t prove the experimental ones would resist it. I have been using ‘Celebrity’ as a control plant. It is an early hybrid that has short plants and resistance to quite a few diseases, though not to tomato late blight. You could choose another control, or could duplicate my choice to stay as close as possible to experiments that have been done already.
The more plants of each variety you can use, the better, since you don’t want an accident, such as a broken stem or a virus attack to wipe out your one plant of a particular variety, and you want to know that your result is true in more than one plant (though because hybrids are more or less identical, what is true of one plant is more likely to be true of another than if you are groiwn heirlooms). I try to have at least 2 plants of each variety; use more if you have lots of space.
Now you will have two goals. The first is to give all of the varieties in your trial as close as possible to the same conditions, so that resistance or susceptibility to late blight is the only difference. If you can’t treat them exactly the same (for example, you can’t grow them in exactly the same spot in the same year) you will randomize that aspect of their conditions. Your second goal is to keep track of what happened and record it. This you will do by making sure you know which plant is which and by recording whether the blight struck each plant over the course of the growing season.
Start seeds for all of the varieties in your experiment on the same day, to ensure that they will be at the same age. Label each kind with a tag that gives the date you planted the seed and the name of the variety. When they have a couple of true leaves, pot them all into individual 4” pots on the same day. Make sure each 4” pot has a label with the date of planting, (date of potting up is optional) and the name of the variety. Don’t lose track of your varieties!
Choose a place to plant your experiment. Give the plants adequate space. We had 3 rows of 4 plants each in 2012. The plants were about 18” apart, the rows 3’ apart. Prepare the soil of the entire area in the same way. That is, spread any organic amendment or fertilizer evenly and then dig it in. If you plan to set up a drip system, set it up over the entire bed so that all the plants will be evenly watered by it. We used a half-inch tubing down the long side of the bed and quarter inch drip line with emitters every 6 inches. We ran two of the emitter lines along each planted row, one on each side of the plants.
Wait until mid-April or early May to plant out your seedlings, so the air and soil have warmed nicely. On the day you plant, have handy a hole-punch, twist-tie, paper and pencil. Before you plant, randomize your planting, as follows:
Choose the seedlings you are going to plant and write the name of each on a small piece of paper. If you have two of each variety, make two paper names for that variety. Fold all of the papers up to conceal the names and draw names at random. (I like to draw them from a garden hat as a sort of joke, but you can draw them from whatever container strikes your fancy.)
Make a map on a note pad. Start at one corner and make x’s to mark where the plants will be. Start in one corner, and as you draw names, write them on the map—progressing across the row and starting on the next row until you reach the end. With your randomized planting, you will reduce the chance that differences from one side of a bed to another, such as shade or wind patterns will throw off your results.
This photo shows the making of the randomly arranged map. I draw variety names from an actual hat, just for fun, but you could use any container. Shown, left to right, are Donna Mandel, Pam Peirce, and Ken Jacobs.
Now plant the tomatoes in the arrangement described on your map. As you plant each one, use the hole-punch to punch a hole in the label and use a piece of twist-tie to fasten the label to the main stem of the plant. be careful not to make it too tight. You want room for the stem to get a little thicker without being strangled.
A labeled plant, with the label attached to the main stem. Make sure the twist tie is loose, so the stem can thicken a bit before you move the label higher on the growing plant. Note driplines on either side of the row.
Water well when all of the plants are in the ground, and water as you would tomatoes normally from then on. Probably best not to sprinkle water on the plants after the first day, but on that day it can reduce the chance of wilting.
As the summer progresses, you will have three goals: 1. Keeping track of which variety is which, 2. Supporting your plants, and 3. Recording what happens vis a vis late blight.
In order to keep track of the varieties, every 2 or 3 weeks we removed the hole-punched labels and moved them higher on the plant. We were careful that they were on a main branch, so it would remain clear which plant each was on, and so that we didn’t lose track of them when a small branch or leaf died. Of course we did have the map we made as a backup, but is nice to be able to see which plant is which at a glance.
In last year’s trial, we supported the plants using the chainlink fence that was at one side of the bed, tall bamboo stakes and jute twine. We tied the twine to the fence, then wrapped it around stakes along each tomato row, then tied it to a stake at the end away from the fence. We used two pieces of twine per row, one on each side of the plants. As the plants grew, we used more twine at higher levels.
Periodically, we observed to see whether late blight had appeared. We didn’t stick to a strictly regular schedule, since we weren’t interested in the exact progression of the disease, but did check every 3-4 weeks, and were able to tell which plants died first and which had fruit set, etc.
Here I am, on August 11, 2011, recording relative damage to the plants. Note the plant label, which has been moved higher on the plant twice to make identification fast. The plant with the label is 'Golden Sweet'. The still-green plant behind me is 'Defiant PhR'.
Need I say that, other than watering at ground level to keep leaves dry, an ordinary precaution I would always take when growing tomatoes, I didn’t take any measures to prevent or avoid late blight? I didn’t put them under cover or spray them with anything. In fact, I grew the trials in locations where late blight was definitely a problem in the previous year, just to be sure my experimental varieties had something to resist.
The Genetics of Tomato Late Blight Resistance
There are two aspects of genetics that affect resistance. I am not going to be able to explain them in great detail here, but here are a few facts. The first aspect is the genes that provide the tomato variety with resistance.
In last year’s trial, I grew 4 varieties I obtained from Johnny’s Selected Seeds. Rob Johnston, the founder of that seed company, told me that two, ‘Golden Sweet’ and ‘Plum Regal’ had one gene that provided resistance to late blight, and they were heterozygous for that gene. That is, it was on only one chromosome of a pair. The other two varieties, ‘Mountain Magic’ and ‘Defiant PhR’ had two genes that provide resistance. They are heterozygous for these genes, meaning that each of them appears on only one chromosome of a pair.
'Golden Sweet' died in my USF planting, but I did get fruit in my community garden, in a warmer microclimate, and where we have been taking out susceptible plants in winter to reduce the number of spores in the garden. The fruits are small, but were indeed sweet.
'Mountain Magic' was alive, with some damage, at the end of my USF planting. It set some fruit there, but only ripened a few of the fruits it set. In my community garden, it ripened plentiful fruit. The fruit is about 1 1/2 inches in diameter and has an average flavor.If you look carefully, you will see a couple of brown stems, lost to late blight damage.
'Defiant PhR' had only minor damage at the USF plantng, but didn't set fruit. However, in my community garden, the one plant had 9 pounds of fruit. Size varied, but the biggest was over 3 inches across. This is a beefsteak-type, meaty fruit, with good, though not excellent flavor.
In my trial, the ones that were heterozygous for two resistance-endowing genes fared better than the ones that had only one of them. Johnston says when varieties have been bred that are homozygous for these genes, that is, have them on both copies of a chromosome pair, resistance should be even better.
I don’t have details about the genetic reasons for resistance of other varieties, such as ‘Juliet’ or ‘Legend’. The only other fact I can relay is that some wild tomato types have resistance through different mechanisms than the above. Examples are some cherry tomatoes and currant tomato, which is a different species than most tomatoes.
In 2011, 'Juliet' died at USF, but in 2009 and 2010, it produced fruit in trials in my community garden. This fruit is smaller than an average past tomato, but larger than a plum tomato. Flavor is good, not great, but production is very good.
The second genetic aspect, the other end of the resistance equation, is that the late blight organism is not genetically uniform. The correct term for the variations are “clonal lines.” They are clonal because they reproduce asexually, thus they are identical to their parent organisms. Why are there different ones? Presumably due to mutations that appear from time to time. There have been 24 clonal lineages in the US, but some may have died out. In any given year, there may be 4 or 5 active ones.
I have made slow progress on tying tomato variety resistance to clonal lines of the late blight organism. I do know that ‘Legend’ resists US3 and US11. And I know these aren’t the clonal lines we have, because ‘Legend’, touted as a breakthrough in late blight resistance, dies first in my trials.
What Clonal Lines do We Have?
When I realized that resistant varieties may have resistance to specific clonal lines of late blight, I saw the importance of finding out which clonal lines we have in the Bay Area. Until recently, I was having difficulty finding this out. The breakthrough this year is locating the website www.usablight.org . This USDA-funded project is a national consortium of late blight researchers. They don’t know what clonal lines we have, but they want to know!
You can help find out by submitting samples of your infected plants. Do this by going to the website and clicking on “reporting outbreaks,” then on “first time registration.” Register now so you will be sent a copy of the APHIS permit you will need in order to submit samples, since they will be shipped across state lines, to a lab in Oregon. Once you have the permit, you will be able to print copies of it to include with your samples. They prefer leaflets that have blackened lesions, but which have not yet wilted, and would like at least five examples in a sample. You register your sample online, so you will get a sample number to write on the sample bag before you send it, and can then use that number to track your results. You also print out a sample submission form to include with your sample, and write the sample number on that too.
Full directions are on the website. I encourage you to look and register now, so you will be ready when the disease shows up.
A List of Tomato Varieties You Might Like to Try
Here is a list of varieties I have trialed, plus a few I haven’t yet, as reported in my Chronicle/SFGate column, follows. (See that article at http://www.sfgate.com/columns/goldengategardener/archive/ for full trial results.) My best results so far have been with ‘Juliet’, ‘Defiant PhR’, ‘Mountain Magic’ and, in a warmer SF microclimate, with ‘Golden Sweet’. As seed sources for varieties listed below, start with Johnny’s Selected Seeds and Totally Tomatoes.
Note: Numbers (i.e. “70-75) indicate days from transplanting to first ripe fruit under normal conditions, though San Francisco plants may be slower. Det=Determinate or short plant. Ind=Indeterminate, or tall plant. These varieties are listed in order of fruit size—smaller to larger. For more information, such as flavor and other disease resistance, see seed catalogs.
*in my 2009 trial, **in my 2011 trial, ***in both trials
*Currant tomato—70-75, Ind, fruits under ½”, avail in red, yellow, and white. This is Solanum pimpinellifolium, a different tomato species that has some resistance.
*Matt’s Wild Cherry—60, Det, fruits marble-sized, red, some resistance.
*Tommy Toe—70, Ind, ½ -1” round red, fruit, some resistance
*Koralik—61, Det, 1” fruit, round red, some resistance
Red Pearl—58, Ind, 1 oz. oval fruit, said to have intermediate resistance
**Golden Sweet F1—60, Ind, oval, grape-sized yellow fruit, moderate resistance
**Plum Regal F1—68, Det, 4 oz., oval, red fruit, moderate resistance
***Juliet F1—60, Ind, 1½-2 oz. oval, red fruit, in clusters, moderate to good resistance
**Mountain Magic F1—66, Ind, 2 oz., red fruit in large trusses, good resistance
**Defiant PhR F1—70, Det, 5-7 oz. round, red fruit, good resistance
***Legend—70, Det, 4-5 inch, round, red fruit, poor resistance in my trials.
Ferline F1— Ind, 5 oz., round, red fruit, listed as “tolerant” of late blight.
Old Brooks--70, Ind, large fruited, said to have “superior” resistance
Please do write me if you have questions or any results to report. You can send me email at firstname.lastname@example.org or write a comment into this blog.