Speaker: Bikash Ghimire, Ornamental Pathologist, Department of Plant Pathology, University of Georgia
Introduced and moderated by: Bethany Harris
Duration: 28:31
Segment: 08 of 08
Series: Cut Flower Symposium
Webinar Date: July 23, 2026
Introduction
Bethany Harris: And so I have the pleasure of introducing our next speaker. And today we’re going to really focus on the speaker. We’ll talk a little bit about disease management. Dr. Bikash Ghimire actually is our ornamental pathologist at the University of Georgia, and he works to support many different types of growers, from greenhouse and nursery growers to cut flower growers.
So he’s going to share a little bit more about disease management and cut flowers with us today. So I will turn over the floor to him.
Bikash Ghimire: Awesome. Thank you so much, Bethany, and the team for this opportunity to talk about some of the disease in cut flowers. As Bethany mentioned, my name is Bikash Ghimire.
I’m a new ornamental pathologist here in the Department of Plant Pathology at University of Georgia. And what a pleasure to be here and speak about the cut flower. And by the way, this is my first time attending the symposium. Here with me.
Production Environments and What Each One Costs
Open Field and High Tunnel
I’m going to start with a couple of the production environment and what are the issues that you might be facing. One, starting with the open field production. If you have a field, then it is weather dependent. You might have conditions with drought, rain, storm, wind, and temperature extreme, which are some abiotic factors.
And then you also have to deal with irrigation. Like you obviously will irrigate the field, but also you have no control because there will be rainfall, and that’s where you will bring all the diseases and pathogens, so which I’ll talk in a minute. And then if we have high tunnel production, oftentimes you have a reduced airflow, there is a high humidity condition and high temperature stress. And also you can control irrigation, so you have a better control compared to the in the field.
And then you can also provide extended seasons, and then the plants are often longer and then are thicker stems and all that. So you have both pros and cons of both environments. But why I’m relating bringing this point is because you’ll have cases where you have disease in one environment versus other, depending on what production system you are in.
Soil, Previous Crops, and Raised Beds
And also talking a little bit about the soil consideration, I know the other speaker already mentioned previously. So for the field production particularly, it matters because what crop you planted previous season is going to affect the way, because a lot of the pathogens, they reside, like, they will have… They reside in the soil or in the debris of the plant, and then they will come back next year.
So that’s why in field production you might be seeing more diseases, and then there are survival structure of the pathogen that can survive for several years, like five years, even 10 years, and I’ll talk about a couple of those in my slides later on. And in the raised bed condition you can have a combination of the field soil and the soilless mix, some kind of compost or organic amendments. And when we say organic amendment, it could be compost, composted bark, or some crop residue, the products, byproducts like peanut hulls and all that. So you can have those organic amendments, and then that is how easily we have this setup in the field.
Disease Management Before Fungicides
Now when it comes to disease management, we have certified organic growers, we have organic and non-organic production, depending on what group you belong to, we have different options. Again, like if you are organic growers, then you have very less options because you cannot do chemical fungicides and you have to go to biological agents or one of the plant extracts, and then… For this biological product, it can only be applied preventatively because it does not have any curative activity as does with some of the other chemical fungicides. That’s why it is very important, and that’s why as a plant pathologist we always emphasize on cultural practice first, because a lot of times these diseases come due to improper cultural practice.
So that’s why we recommend cultural practice and also planting resistant cultivars whenever possible. But sanitation is very important, roguing infected plants, scouting early in the season, and then doing a crop rotation to avoid some of this inoculum buildup in the soil. And of course, there are irrigation management, because a lot of time due to overwatering, we’ll have a problem of some of the root-borne issues. And then fertility management is other important aspects when it comes to disease management. So basically disease management, you are taking into account a number of factors, even the crop production as a whole, so that you don’t have to apply fungicide, or if you have to, then apply at a minimum dose and rate.
The Diseases of Specialty Cut Flowers
So… When it comes to disease in specialty cut flowers, there are numbers of them. I have listed on the left. You can see root and stem diseases, and it has a mostly wide host range because they are in the soil.
That’s why they can infect different plant types, and then some of the common ones that we often identify are Phytophthora, Pythium, Rhizoctonia, also causing white mold, we have Sclerotium rolfsii causing southern blight, and we have Fusarium stem rot. On the foliar side, as on the right, mostly they are host specific, means they can only go and infect particular type of plant, or even sometimes particular cultivars. And then some of the common ones we see are leaf spot caused by bacteria, by fungi, leaf rust, powdery mildews, and we have Botrytis blight, which is also a big problem, and some virus diseases, which I’ll talk about this one by one in my next slide.
Where the Pathogens Come From
So where does this pathogen come from? What are the source for this pathogen in the coming in your facility. So a lot of time it comes from the infested seed, like for example for downy mildew, bacterial leaf spot, you might have that infection going on in the seed, or the seed lot is bad, and that’s how you are getting this disease in your planting stock. And then also you might be bringing infected plants from outside, and that’s where a lot of time you’ll be starting getting those problems, and oftentimes you also see that the plant is asymptomatic.
You don’t see any symptom in the beginning, but over the time they might start producing symptoms and then become a problem. And then also contaminated trays, pots, tools and pruning equipment, benches, all this could be the source of inoculum. And then as I mentioned already, the crop residue from previous years could be a problem. When the previous speaker was talking about the crop residue, she was mentioning about how it should be non-disease or pathogen free, so that’s why we have to be very careful when you do a crop rotation and all that thing.
Wind-blown and rain-driven spores are also other source, and then you’ll be surprised, like how many times there are… There’s no problem in the field and you overlook, and then over the years, if you come back after two, three, four years, then I’m sure you’ll get a disease. It will be at the time, if you don’t control or have an action in the bed from the very beginning, then by the time it might be too late, and you might see a big epidemics over there. Just keep that in mind.
Root and Crown Rot
Reading the Symptoms
So now let me start with root and crown rot disease. So it’s very confusing sometimes with this kind of symptom because you will not know if it is abiotic stress or it is due to the pathogen. And then oftentimes when you go out in your tunnel house and you see these patches like where plants are dying, and then you see these patches are even like a plants are chlorotic, they are yellowing, and then all that wilting sign, then you need to take a closer look and try to see the entire plant just to see what where the problem is.
And a lot of time you’ll see the problem is in the root and crown region. And then as I already mentioned, you can see this chlorotic, yellowing of the leaves, wilting and all that. So you have to carefully pull the entire plant, including the root system, and then in the root, then you’ll be able to see some kind of this, you know, reddening or some kind of discoloration. And then you will also see the problem in the root system.
Water Molds, Sporangia, and Zoospores
And on your right you can see this pathogen, as you are seeing here in the little video, like here is a pathogen producing sporangia, and inside that sporangia, [VERIFY: "copseps"] sporangia, it has these zoospores, which are the motile spores. They need water. That’s why they are called water mold. And these spores, when it gets water, then it can freely swim and then go to the healthy root system and then creates a problem.
So that’s why if you have overwatering condition, then that’s where most likely you will see root and crown rot disease. And as I mentioned, the root system will be like soft, it will slough off, and you can also smell some bad odor, and that’s how you can identify the pathogen and the disease behind the scene.
Managing Phytophthora
What, how, when it comes to disease management for Phytophthora. So as I mentioned, water management is the key because if you have overwater condition with bad drainage, like a poor drainage condition, then that’s where you will be seeing a lot of problem. That’s why when you have a lowland, when you have areas where there is water stagnating, then that’s where you’ll see problem arising. And for the management side, you can incorporate organic material that can help the soil and also the drainage condition.
So whenever you see the disease plant, then immediately you have to remove the infected plant as a whole, including the root system. And then you can do some kind of, you know, the treatment in the beginning when you start, even before planting, you can do some kind of the biological, apply some kind of biological products, you can do soil solarization, and all those things that will, even use the steam, that will help to inhibit the pathogen. And there are some fungicides that can be applied when it comes to transplanting some of these plants in the container.
White Mold, Sclerotinia sclerotiorum
Now let me talk about this another important disease called white mold, which is also known as Sclerotinia stem rot caused by Sclerotinia sclerotiorum. So this is a lot of mouthful of scientific name here of the pathogen. But anyway, for this pathogen it loves cooler temperature, high humidity and a lot of moisture, as it does with the Phytophthora that we just talked about. So it can affect numerous crops and flower species, and especially the cool season crops are the ones that get infested.
And then when you see that white fungal growth can be seen in the stem, and then you can see the stem lesion, which is like a tan color. And that’s how you start the symptom, like with some kind of the lesion in the stem, and over the time that you can see this white fuzzy mycelial growth. So it can also produce the overwintering structure called sclerotia, which I’ll show in the next slide. Oftentimes when it gets infected, then the the stem is soft, mushy, and then the entire plant kind of collapse. So when you see that, then first of the thing that you need to do is remove that infected plant and make sure that it does not go to the compost, but rather you safely dispose it out.
And the overwintering structure that I was mentioning is called sclerotia. So this is a black structure that you can see over here. And then you can also see this structure. This is just like a black, the mycelia that are clustered together, that hyphae, and forming this pigmented structure so that it can thrive harsh condition and can remain in the soil for several years, and some of the papers and literatures have mentioned that it can survive for five years or even more in the soil. So that’s how it can be a problem even after several years of crop rotation, if you are planting the susceptible cultivar, and the environmental condition is favorable for the pathogen.
Southern Blight, Agroathelia rolfsii
So the next disease that we have in the list is southern blight, which is caused by Agroathelia rolfsii. It used to be called as Sclerotium rolfsii. So compared to the white mold, it favors warm condition instead of the cool condition. But the other environmental condition are pretty much similar. It loves humid condition, a lot of moisture and all that, which is perfect now in the summertime. So that’s where you see more of the problem of this southern blight, and it can affect different kind of ornamental and flowering crops.
So it produces this when you see on the crown region that you can see the hyphae, that white radiating hyphae, and also you can see this brown color mustard-like must-, it looks like a seed of the mustard, and then that is the sclerotia of the pathogen. That is again the overwintering structure that can remain in the soil for years. For the management practice for the southern blight, so all you have to do is you dig there and remove the infected, the disease plant, and then what you can do is you can dig soil eight centimeters deep and then you can flip the soil and bury that sclerotia deep enough that it cannot come out. But again it can remain but cannot come out and be a problem.
And do not add infected plant to the compost pile, because that could again, like when you use the compost next year, then that overwintering structure likely come back and start the infection. So some of the fungicides are there which can be effective, but it does not help the disease plant. It can only help the non-infected plant or the new growth that it can cure. It does not cure basically. It has to be applied preventatively. That’s what I mean to say.
And then when you do the plant propagation, you bring tubers or corms in your greenhouse or in your tunnel house, then you need to be very careful, like you don’t introduce those pathogens in your planting materials to your facility. So that’s how you control this disease.
Fusarium Stem Rot
So the next root-borne issue that we have is with Fusarium causing stem rot and wilting, so which is one of the biggest problem I would say that has been reported out in lisianthus. And then you can see how in the stem, with the stem rot, you can see some kind of the lesion in the stem, in the crown region, and then you see wilting of the plant. So when you see those stress, like a wilted plant, this kind of the problematic, the plant in the bed, then all you have to do is remove that as early as possible, because you can spot in the early stage when you do a scouting, and then that will help to disseminate the pathogen to the next healthy plant. So I think that’s pretty much I have for Fusarium.
Botrytis and Gray Mold
So the next one I have in the list, and let me check my time. So the next one I have in my list is Botrytis. So which is which cause gray mold, so which is a big problem, especially it can be in the standing plant, and also it can be in the harvested flower, once the flower is harvested, and then it can also be a big problem if it does not get the proper storage facility, then you can have that problem of gray mold. So it can affect numerous plant species.
Again, it loves wet, humid and moderate temperature, and it can colonize the succulent plant part, like a flower part or the young shoots on the top. And then in the beginning it starts with a leaf spot and some kind of the blighting in the flower, and it can cause stem canker, it can cause crown rot and all these different kind of the issue. And then this is the beginning, this will become a big problem. And then as I said, like you can see over the time then the entire flower will be covered with this mycelial mass. And then also it produces spores very quickly, and that spores is windblown. So this is the spores that is in a cluster that is oval shaped, and that spores are when the humidity is high above 85%, that spores can be windblown, or it can be splashed with the overhead irrigation or whatever the irrigation, like or the rain, even the rain event, and that can be disseminated to the next healthy plant or next field, and then that how the infection is starts actually.
And you can see the fuzzy growth on the tissue and all that, and you can easily identify for Botrytis. This is one of the easy diseases to identify in the field. And as I said, it is a problem in the post-harvest and storage as well, that causes a lot of economic issue if you don’t properly store the harvested flowers and bloom. You need to remove the infected plants in terms of the management, and then also the stem that has this deadhead, whenever you see some of the flowers are dying or decaying, then you want to better remove that because that could be the source for pathogen, an entry point.
So providing good air circulation is very important, and again proper spacing, as speakers previously mentioned about how spacing is important. So it is also related to disease management. So you need to provide proper spacing so that we have a good air circulation and avoid that high humidity accumulating in the leaf surface. And then there are some fungicides that could be applied for reducing the infection and the growth of the pathogen.
Powdery Mildew
Next one in the list is powdery mildew, that requires warmer, humid but dry condition, and it can affect a lot of different plants, but different species might be you might be seeing the same powdery mass on the top of this leaf, but there might be different kind of species of the pathogen for each individual host type. So they are very much host specific, that’s why. So the pathogen that is in sunflower might not be the same in zinnia, though the symptoms and the signs of the pathogen look very much similar.
And then as with other disease, then if you spot this in the early stage, you remove the infected plant, but at the same time you can apply fungicides. Some of the fungicide that we can apply for powdery mildew and even leaf spot that I’ll talk in a minute, like some fungicides on FRAC code 3, DMI fungicide, QoI fungicide, azoxystrobin in group 11, or some of the group 7 fungicides are very effective for controlling this disease. And then there are some biorational products like MilStop is found to be effective in powdery mildew to stop the spreading of the disease.
Downy Mildew
So the next one is downy mildew. So with the downy mildew you can see that you see some kind of stunting, flower distortion and some on the top, on the above surface of the leaf. But if you flip the leaf and look underside, you can see the white spore production underneath, and that’s how you identify this disease, and it’s a problem in a lot of ornamentals, especially sunflower or [VERIFY: "bacia"], that you can see a lot of issue with the downy mildew. And this is a systemic disease that goes from the vascular region, that flow, that kind of pass in the vascular region and cause the infection.
Leaf Rust
So the next one I have in the list is leaf rust. And one of the common ones that I see is in the snapdragon. As you can see, snapdragon, you can see this yellowish discolored patchiness on the top, and when you flip the leaf again on the underside, you can see a lot of this kind of, you know, pustules that has a tons of these spores, if you see under the microscope.
So these spores are windblown, that can easily be blown by air, wind, and can go from several kilometers and then miles and can cause disease. So rusts are very host specific, and some require multiple host type, like two, at least two host type, to complete its life cycle, but some of them only require one, like in the rust in sunflower or snapdragon, they don’t require another host type to complete the life cycle. But anyway, so when you see those plants, again, like removing the infected plant, and you’ll hear again and again from me talking about the same management practice of early scouting and removing the infected plant, because that’s the first step of disease management. And then of course we have fungicide that can be applied preventatively to control disease.
Leaf Spot Diseases
Fungal Leaf Spot
So the next one is fungal leaf spot. So which is, there are so many leaf spots that it’s not convenient to list all of them here, but just as an example, in zinnia, whenever I go out in the field I see a lot of problem with Cercospora leaf spot. So that’s a big problem in zinnias, and you can easily identify because you can see this tan center and then this is surrounded by this purple margin or border. And then you can also see the fungal fruiting structure of the pathogen.
So for the leaf spot management, again, infection and spread is favored by leaf wetness, high humidity, poor air circulation, and then the fungi can overwinter in the plant residue. So that’s why it’s very important to remove that extra debris, and then the diseased or the dead stem and leaves that need to be taken out from the field so that it does not be a source of inoculum. And then also important point is to keep the foliage as dry as possible so that you don’t have that extra leaf wetness.
And spores are water splashed, so that’s why if you are doing overhead irrigation, you might want to cut off that and go to the manual irrigation so that you don’t help in the dissemination. So again, the pathogen is host specific. That’s why rotating crops one after another year might be helpful in terms of controlling the disease. And then we recommend applying some of the biological and chemical fungicides that I mentioned. So that was the fungal leaf spot that I just mentioned.
Bacterial Leaf Spot
And then we have bacterial leaf spot, which is different from fungus because for bacteria it produced this angular leaf spot, which is compared to fungus, they produced in the concentric ring or around the structure. Here it is angular and it is in the vein region, it is like going in the vein. And then you can see a halo, which is called the yellowing around that leaf patch or that spot. So that’s how you identify fungus versus bacterial leaf spot, because the management for bacteria versus fungus is a little bit different in terms of applying fungicides.
So for bacteria, mostly we recommend copper-based fungicides like mancozeb. Those could be very effective in terms of controlling this pathogen. So again, water management is very important because if you have too much leaf wetness on the foliage, then that is when the bacteria starts infection.
So removing the infected plant, and then also one thing to note is the bacterial pathogen are seed-borne. They can be in the seed and can go pass to the next generation. That’s why it is important to make sure that you do not save the seed from the infected plants for the next year.
Virus Diseases and Diagnostics
The Viruses and Their Vectors
So the next one that I want to quickly touch is virus disease. So there are several virus, plant virus diseases that are a problem, and just to list a few of those, we have this INSV, Impatiens Necrotic Spot Virus in snapdragon, we have Tomato spotted wilt virus in zinnia, and these are just like some of the common virus that we often identify. And virus are spread by insects like thrips, aphids, mites, and it also is spread by grafting and propagation.
That’s why the only way to control or manage this disease is by either applying insecticides to control insect, or just avoid propagating the infected plant. But there is no other cure method for this virus diseases, that’s why whenever you see the plant, spot the plant getting this virus, then you just have to uproot the entire plant and safely dispose them, and then also control, the monitor the insect population, and apply some kind of insecticides to control this vector that transmit this virus. So that’s the only way you can see, and as you see the virus diseases are very easy to spot, because you will see some kind of the ring spot, like a mottling, yellowing of the leaves, and this unusual pattern, when you see, then you are mostly likely dealing with virus diseases.
ImmunoStrip Testing
So luckily we have this ImmunoStrip that is from Agdia that we can test, and the result can come as quick as five minutes. And then all you have to do is take that infected leaves, macerate it, and then throw it at the ImmunoStrip in the buffer bag. And this is just like a pregnancy test kit where we have two control, two lines. When you see both line, then that is positive. When you see only one, that is negative.
For example, here in this case with this plant, zinnia, sorry, we have this TSWV positive for zinnia, whereas INSV is negative because there is only one band. So that’s how we identify the disease right away. And there are other ImmunoStrip that is out from Agdia, which is for TMV, CMV, and then we have for Xanthomonas, Phytophthora, and Rhizoctonia, which are also very helpful in terms of quickly identifying the pathogen, and then it can also be taken to the field, so we can do either in-house testing or in the field, and then it does not cost that much, like a six to eight dollar per ImmunoStrip, so it could be very handy resources for growers.
Biological Control Products
So this is, I think, I’m towards the end now. So I know I’m rushing a little bit, but just to wrap up my, or cover all of my content, I was a little bit faster. But these are the list of some of the biological control as, and that could be applied. It is listed for edible greenhouse, but it can be applied for ornamental plants, and I have tested or I’m about to test some of those, like I have tested some in the turfgrass, but I’m about to test this in the ornamental, so some of the Bacillus products are very effective in controlling a lot of different diseases, foliar and also the root and crown rot disease.
We have some Trichoderma products that is also very effective in terms of controlling, you know, Fusarium, Pythium, Phytophthora, and all that root and crown rot. And of course we have neem oil, we have some Pseudomonas, and then plant extract, so that can be applied.
Resources for Growers
So… With that, just I want to leave you with this resource. e-GRO is out there, which is an online platform where it gives you a lot about disease, insects, environmental and physiological and nutritional disorders and all that. So you can utilize these resources.
The link for the website. Just if you have not gone through this, this could be a useful resource for the growers. And then we have Georgia Pest Management Handbook that has a very comprehensive list of fungicides and application amount and dose and intervals and all that you can find. So we always recommend referring to this handbook whenever you have to apply any fungicide.
So with that, that should be all for me. Thank you so much for patience and listening to me, and then I’ll be happy to take any questions either now or in the future. So here’s my contact information. Thank you.
Questions and Answers
Aster Yellows in Coneflower
Bethany Harris: Bikash, thank you so much for sharing with us a little bit about disease management strategies this morning. We do have a couple questions from the audience, and the first one is, can you speak on aster yellows and prevention? This grower noticed their first coneflower infection this week. So do you mind sharing a little bit more about that?
Bikash Ghimire: So aster yellows, I think we got an email about aster yellows, I remember. So it’s a bacterial kind of disease caused by phytoplasma. And then so phytoplasma is similar to bacteria.
So again, with this kind of pathogen disease, it’s very, sometime it is confusing with, I remember I had an email where sometimes it is confusing with the herbicide injury that it produced this, you know, witches’ broom kind of symptom and all that, that is very confusing, but, but this, whenever you see this disease, all you have to do is just uproot the plant and then safely dispose, because it can be trans-, I believe it is transmitted by some kind of hoppers. I forgot what exactly that is, but that’s why you safely dispose the plant, and then just make sure that it does not go out and spread on the healthy plant, and also you can apply some kind of insecticide to control.
Bethany Harris: Perfect.
Botrytis in Tulips, Year to Year
And I think we have one more for you too. If there is a widespread of Botrytis infection in tulips. Will it remain in the soil until the next spring? And what can be done to prevent it from infecting tulips the following spring that are planted in the same area?
Bikash Ghimire: That’s a very good question. Yes, the pathogen can reside. The picture, if you remember I showed, so it was, so it produced a lot of conidia in the finger-like appearance, and then that remained in the in the soil debris, or in the, not necessarily in the soil, but in any kind of plant debris.
And then it can come back next year depending on what type of plant you are planting. If you are planting the same host or same plant or cultivars, then that could come back again and then start infection. So I would suggest if you see those in the early stage, make sure that you scout that all the plant in the early stage. And then for the next season, you might want to start with some kind of the application in the beginning if you are planting in the same bed. So I would suggest going and applying any kind of, you know, systemic product.
You can start with some kind of contact fungicide first, like chlorothalonil, fludioxonil, and all those are contact fungicide that is it can, that has a preventative action, mode of action. And then you can also apply some kind of, you know, the systemic fungicides like DMI fungicide like tebuconazole, azoxystrobin, and boscalid, those could be applied. So you can do one early stage application as a contact and the next one as a systemic, depending on how severe you will see the symptom.
Bethany Harris: Perfect.
Bacterial Stem Rot
And then I think we’ve got one more last question here. And it’s, do you have any recommendations for bacterial stem rot?
Bikash Ghimire: Okay. So for the bacterial stem rot, I know somebody’s asking about the Pectobacterium. James is asking about Pectobacterium. So for the Pectobacterium, I would suggest going with, for the bacterial disease, I would suggest going with copper-based product. Mancozeb could be one of the effective ones to start with.
They have a better control compared to other the regular fungicides for fungal leaf spot, fungal, for example Fusarium stem rot, which is also a big problem in a lot of other ornamentals. So I would start with the copper-based fungicides. So that could be the best bet.
Bethany Harris: Thank you very much. And thank you all for being here today, joining us this morning.
Transcript processed for UGA Center for Urban Agriculture / Cut Flower Symposium Archives
Source: Corrected SRT (Stage 1), GTBOP_Transcript_2026-07-23_GhimireDiseaseManagement.srt (315 blocks)