Fireblight
Pomological Solutions #3
Fireblight, Erwinia amylovora, is without a doubt the bane of many an apple and pear grower. Fireblight is a bacterial disease that once systemic (inside of) the tree, it is nearly impossible to eradicate without radical tree surgery. It also affects quince and to a lesser extent many species in the rose family (Roseaceae spp.), such sa hawthorn (Crataegus spp.) But, to be fair, it doesn’t affect all apple or pear equally. In fact there are numerous species and varieties of each that are highly resistant and even immune. But to anyone growing traditional and especially late blooming apple or pear varieties, fireblight can make growing those fruit a pointless adventure. Yet there are varieties, practices, and treatments that can be used to outsmart and outfight this nefarious disease.
Today, fireblight is a disease that affects apple, pear and quince growers worldwide, but it was not always the case - at least not to the degree it is today. It was first reported in North America in at least 17801, but did not become a major problem until the 20th century. Outside of North America it was not first reported until the 20th century, suggesting that it is native to North America and why many native Malus spp. are not susceptible to this disease2. As apple and pear production spread from east to west, so did the disease, creating widespread susceptible hosts areas (e.g., orchards) throughout the continent, especially in major production regions like Michigan and Washington State. In the 1960s the Hudson Valley NY pear industry was practically wiped out by fireblight, prompting many growers to switch to less susceptible apple cultivars. In the 2000s, as hard cider starting growing in popularity, many started planting popular French and English cider varieties that for better and worse3 are extremely susceptible to fireblight. Infections can happen throughout the year but are most likely to happen during bloom and after traumatic summer weather events (e.g., hail).
Fireblight’s Life Cycle. Once in a host plant (tree), fireblight inoculum overwinters in cankers on infected trees. When the sap starts to rise in the spring, the inoculum-laden sap oozes from the margins of these cankers to be splashed around by wind and rain onto the trees. These epiphytic bacteria then grow and spread as warmer, moister spring weather prevails. Yet they do not create new infections unless there is susceptible tissue and conducive conditions. Cold weather doesn’t necessarily kill the bacterium, but may slow down its growth. Warmer weather and spring rains are all it needs to grow. As the trees approach bloom, the temperatures are getting warmer still and rainfall (even dew) becoming more frequent. A combination of receptive tissue (e.g., flowers), warm temperatures (60-85F) that lead to an adequate accumulation of degree days, rain or dew, and active inoculum are all that are needed for a lethal fireblight outbreak. If any one of those conditions is not right, it is very unlikely a fireblight outbreak will occur.
What to do? Choosing less susceptible varieties, if you have that option, are without a doubt the first best route to take. But if you already have an orchard of susceptible in the ground, then your options are limited to cultural/sanitation and treatments.
Cultural/Sanitation. If you have or suspect you have an infected tree, cutting or pruning out the infected tissue and burning or throwing it away is necessary. Do not leave infected prunings in the orchard as they can be a source of inoculum. And always cut at least 12” below any visibly infected area to make sure you capture as much of the infection as possible. This, of course, is nearly impossible if the infections are already in the trunk or major scaffolds. In these instances you need to actually carve away the infected tissue and remove it as you would infected prunings. Always carve down to clean greenish-white tissue. Any off color or brown tissue suggests it is still infected. I like to paint or spray on a dilute copper solution to help kill any residual bacteria. Do this when the weather is dry and warm so it will seal over more effectively. Leave the wound open to dry out for at least 24-48 hours before applying any tree paste4.
NOTE: I have attempted using garlic injections to systemically move a natural antibiotic (garlic’s allicin) into infected trees. From the ground up rather than top down was the theory; catch it before it gets any further. The results were not promising, but the trees themselves were severely infected before we started the project.
Blossom Protection. The first major period where fireblight infections can occur is during bloom. As soon as blossoms (flowers) begin to open, they are susceptible to infection. When flowers open, the pistil or female part of the flower is where the inoculum can enter the tree. The bacteria lands on the pistil, grows down into the ovary5, and expands its presence from there by moving into the vascular system and then throughout the trees. You still need appropriate temperatures, moisture, and inoculum levels for infection, but as long there are open flowers, assume that your orchard is at risk.
I recommend treating the pre-bloom period as the period where you can knock down the inoculum load (since you can’t of course do much about temperature or moisture). Pre-bloom protection starts in the dormant season with copper sprays and/or sprays with other materials that can knock down the inoculum-laden ooze mentioned above. Then, as the buds first start show a little green all the way until Pink, regular applications of Double Nickel, Cueva, and Regalia go a long way to keeping the ever-growing epiphytic populations at bay so there is less active inoculum when bloom finally gets there.
Blossom Protection. Your pre-bloom activities, while effective, were probably not a complete eradicant, meaning there is still some active inoculum on your trees as well as in the untreated surrounding untreated areas.
Figure 1. Some novel biological sprays for fireblight control (but not eradication). Know Your Roots LLC, 2020
And so protecting the flowers as they open is critical to fireblight protection during the entire bloom period. Each of the materials listed in the chart above (Figure 1) are effective against fireblight. Some are effective differently, meaning their modes of action are different and they may not have the same level of effectiveness. That’s why it is important for organic growers especially to employ multiple strategies starting with not growing susceptible varieties if you can help it. But we want to stay away from materials that might damage the flower and negatively impact fruit set (e.g., don’t apply any oils during bloom).
SAR - Systemic Acquired Resistant: these products stimulate the plants primary acute immune response systems. They increase the plant’s ability to fight off infections. These products may also have some level of anti-fungal or anti-bacterial activity, but this isn’t how they primarily benefit the tree.
CC - Competitive Colonization: by putting beneficial microorganisms onto the surface of the flower before the fireblight bacteria arrive, the beneficials out-compete the fireblight before they have a chance to take hold. There is also likely biological biochemical competitive activity going on, but primarily it is competitive colonization. The same effect can be achieved - though not as effectively - by boosting the plant’s existing microbiome and/or boosting its population fitness through nutrition or other means.
Stigma - this product creates an inhospitable environment on the stigma (the top part of the pistil) of the flower meaning that even if the fireblight bacteria get that far, they’ll be taking a literal acid bath.
Antibiotics (Antib) - these products have a more traditional antibiotic effect, meaning they kill the bacteria as opposed to a more novel mode of action such as competition. They are not the same as say streptomycin or kasugamycin, both commonly used to fight fireblight in conventional agriculture because they work through different sometimes multiple pathways, but not entirely dissimilar.
Phage - Phage products are still relatively new in the market. How do they work?
From Omnilytics own web site: “AgriPhage uses naturally occurring bacteriophages to selectively infect and destroy Erwinia amylovora, reducing disease impact and crop damage. This targeted approach minimizes the need for chemical treatments, aligning with sustainable agricultural practices.” You can go to their web site to learn more about this promising new approach to fighting fireblight.
Most of these products need to be applied before or at least during an infection. Only the conventional streptomycin really has any retroactive control potential, and even then 24-28 hours after an infection at that. Of the organic materials listed, Agriphage might have some post-infection control, but the reality is that you need to be on top of things before an infection happens to prevent an infection.
NOTE: Always consider your own conditions and threat levels. Read the label (Always - its the law) before using any product. And be both prudent and realistic in your approaches.
Prediction Models. There are several prediction models out there, all of which work from the same general foundation but differ in the nuance of the algorithm. NEWA, or Network for Environment and Weather Applications, is a FREE web site with a wide range of weather and situational based protocols that can help any grower. By tracking weather, crunching numbers, and running tested models, NEWA can help you to predict when an infection is headed your way. Here is a direct link to the NEWA fireblight model page - HERE.
In essence, you, the grower, input some variables or information based on your conditions. Always use the weather station closest to your location if you don’t already have one on site.
Choose a weather station
Choose a first blossom open date
Indicate whether there was fireblight in your neighborhood last year
Choose whether you sprayed streptomycin
Once you’ve done this, then the model will tell you whether or not there is fireblight infection event upcoming and how bad it may be. The data you enter will need to be updated each time you run the model - its only as accurate as the input - especially if you are treating varieties that are opening at different bloom times.
The other model I like is RIMpro. RIMpro is a paid subscription service, but it can run off of your weather station. See graph below. The top (green) graph shows the endophytic population growth. The middle shows the infection threat threshold. The bottom (orange) shows the epiphytic population growth which illustrates how fast E. amylovora is growing on the surface of your trees. And the bottom blue area illustrates the wetness periods. You can see how when conditions prevail fireblight goes from an existential threat to a very real on as it crosses into the orange zone. Here is a link to RIMpro’s web site - HERE.
In both RIMpro and NEWA you can indicate when you’ve applied a treatment. However, this is usually synced to how strep sprays would impact bacterial populations; it won’t be the same for biologicals. Nonetheless, you know that once you’ve applied a treatment it will have a positive effect on the susceptibility of your trees. However, and even with strep, that effect wears off in time so you need to remain ever-vigilant. What these programs don’t do it is enter the relative varietal susceptibility of your trees.
Figure 2. RIMpro Fireblight model illustration (example).
Biodynamic Treatments. Biodynamic treatments by their very definition are not designed to “treat” a problem. They are intended to influence the conditions in the orchard, in effect creating an environment where fireblight becomes less of a problem. So what are those conditions? As we’ve already discussed, temperature, moisture, receptive tissue, varietal susceptibility, and inoculum are the five variables affecting fireblight potential. Given that we have little control over inoculum levels in an untreated orchard of susceptible varieties, how do we affect temperature and moisture biodynamically?
Sequential Sprays - There is a sequence of sprays one can use to alter the weather patterns leading up to bloom. These are the sequential sprays. These can be used in any instance to help change the conditions that may lead to infections. I will post these in detail in another newsletter very soon. But suffice it to say that the intention is to slow the accumulation of degree days via cooling and reduce the potential for moisture via drying. The use of barrel compound, BD500 (horn manure), BD501 (horn silica), and BD508 (Equisetum arvense) are all used in varying sequences to influence cooling or drying conditions. The use of milk and honey (for their respective lime and silica influence) are also used by some as the final sprays in sequential spraying. The sequential sprays ideally happen over a 3 day period that coincide with the proper biodynamic calendar days, but can be condensed into two days (again more on this in another newsletter). The sequential sprays have been shown to work in many situations, though there isn’t any proof they’ll work specifically for fireblight - so just be aware.
NOTE: the barrel compound and BD500 while used for their earthly esoteric influences in these sprays also bring very real biological elements into play, meaning that the practical benefit of the sequential sprays leading up to bloom is to also build populations of beneficial organisms on your plant before an infection occurs.
In the sequential sprays I mentioned the need for drying conditions. This is largely associated biodynamically with light and warmth on flower or fruit calendar days. These elements come from the BD501, BD508, and honey compounds - each of which can also be used individually prior to and leading up to an infection event. In esoteric terms we refer to these as having an astral or warming/drying influence. Other biodynamic preparations having astral influences would be dandelion (BD506) and valerian (BD507) because of their association with an outer planet (Jupiter and Saturn respectively). As well, BD505 (oak bark) can be used to counteract the watery influences of the moon. And while not bringing an astral influence per se, its influence on moisture in this case could be beneficial.
Ideally, you are using these biodynamic treatments in conjunction with a more traditional holistic approach. Starting with resistant genetics, then creating resiliency (strong immune system) through nutrition and fertility, a robust biology above and below ground, and finally relying on effective holistic, albeit allopathic, controls should bring you the control you are looking for. This way you are not relying on any one path of treatment, but instead using a robust approach.
Suggested Fireblight Management Program
1. Dormant Season. Remove any infected tissue – this includes fruit – from the orchard and destroy or compost it. If you have fireblight strikes during the summer you can cut them out and leave them on the ground to dry. Just be sure you don’t drag them through the tree and infected other parts. In severe situations it is best to just leave the tree alone until the dormant season or remove it entirely from the orchard. Blossom blight strikes can be removed immediately when you see them. And in the winter, carve out any cankers you see on trunks or limbs and burn them in a very hot fire. All material should be thrown away or burned in a very hot fire.
2. Silver Tip. Apply Copper to kill overwintering inoculum in cankers.
3. Greentip -> Pink. Apply biofungicides, nutrition, and biology every 5-10 days to increase protection and enhance self-defense mechanisms. The use of the biodynamic preparations and the sequential sprays can start to be used in the lead up to bloom and depending on the weather conditions6.
4. Pink. Beginning at Pink or at the latest 10% full bloom, you can begin application of a biological (such as Blossom Protect) to begin the process of protecting the floral surfaces as the bloom period begins. These sprays should continue through bloom and until petal fall as long as there is susceptible tissue available to the pathogen. Continue to use the biodynamic sprays, but be mindful of the effect the stronger silica sprays can have on flower and pollen viability if applied under the wrong conditions7.
5. Bloom. This is the critical time to protect against any fireblight infections. Monitor infection periods, maintain coverage especially during high risk periods, and if using a biological – don’t let it dissipate. Here is where the biologicals as competitive colonizers can be most useful, especially Blossom Protect. But they only work if they are on the tree and covering the flower surfaces.
6. Summer. When you get into summer and if your tree’s suffer a trauma blight situation, the application of Double Nickle and Cueva may be effective in reducing infections by reducing inoculum, especially ahead of a storm but also after, reducing its spread afterwards, and just providing general disease protection. Summer fireblight infections can turn traumatic quickly if not dealt with right away. Be ahead of the game here. Because of the summertime heat and moisture, bacteria grow faster than when it is cooler, such as during bloom time. It is uncertain how the biodynamic sprays would perform with trauma blight (as it is called) but certainly using 501, 508, and perhaps even 507 to usher in warming and drying conditions couldn’t hurt. Just mind the precautions of applying these preparations (esp 501) at the wrong times.
Fireblight doesn’t need to be a death knell for your trees or your dreams. Simply be smart and act accordingly. With any product you use, read the label or understand how what you’re using or doing is appropriate to your situation. Doing the wrong thing or applying something inappropriately may do more damage than doing nothing at all. And don’t use the kitchen sink approach, you’ll clog the drain!!
Some pinpoint it global ground zero to West Point in New York’s lower Hudson Valley. But the reality is - who knows?
If Erwinia amylovora is native to North America and since NA’s native Malus spp are resistant, this suggests an evolutionary pathway that weeded out the susceptible species or subspecies while the hardy remained.
Worse because they are susceptible to fireblight, better because their late blooming characteristics give them a better chance of surviving a late spring frost.
I can’t ever recommend applying a sealant as they can lock in any infections and prevent the plant from properly healing. If you do apply something, then biodynamic tree paste or a product like Viti-Seal might be the most appropriate.
The same path the pollen germ tube takes when it pollinates a flower.
Do not use the biodynamic sprays as a “control” as you would an allopathic spray. these are intended to influence the environment and the plant itself to create resilient conditions not conducive to fireblight or other pathogens.
What are the wrong conditions? Do not apply silica, especially 501, after the sun has risen above the horizon. The same for equisetum and valerian. Remember you are influencing the conditions in the orchard, not trying to kill anything. You want to bring light and warmth before they are needed.