Segment Title: Disease Management in Specialty Cut Flowers
Segment: 08 of 08
Webinar Date: July 23, 2026
Speaker: Bikash Ghimire, Ornamental Pathologist, Department of Plant Pathology, University of Georgia
Introduced and moderated by: Bethany Harris
Duration: 28:31.0 (1711.018 s, measured)
Series: Cut Flower Symposium
CEU Categories: [PENDING]
NARRATIVE SUMMARY
Bikash Ghimire is new to the ornamental pathologist post at the University of Georgia and new to this symposium, and he opens by sorting cut flower disease by where the crop is grown rather than by pathogen. Open field production runs on the weather: drought, storm, wind, temperature extremes, and rainfall the grower cannot switch off, which is how pathogens arrive. A high tunnel trades that for reduced airflow, high humidity, and heat stress, but hands back irrigation control, a longer season, and longer stems on thicker stalks. Field ground carries a second cost, since what grew there last season survives in soil and crop debris, and some survival structures hold for five years or ten.
Fungicides come late in his order of operations. Certified organic growers have fewer chemical options and lean on biologicals and plant extracts, and biologicals work preventively rather than curatively, so the cultural side has to carry the load: resistant cultivars where they exist, sanitation, roguing infected plants, early scouting, crop rotation, irrigation management, and fertility management.
He then walks the diseases. Root and crown rot reads as chlorosis, wilt, and patchy dieback until the plant comes out of the ground and the roots show discoloration, softening, and a bad smell. Water molds produce sporangia releasing motile zoospores that swim in standing water, which is why overwatering and poor drainage predict the disease. White mold (Sclerotinia sclerotiorum) favors cool wet conditions and leaves tan stem lesions under white mycelium and black sclerotia. Southern blight (Agroathelia rolfsii, formerly Sclerotium rolfsii) wants the opposite temperature and drops sclerotia that look like mustard seed; his fix is to dig, remove, and bury the sclerotia eight centimeters down. Fusarium stem rot he calls the biggest reported problem in lisianthus. Botrytis blights standing plants and harvested stems alike, sporulating above 85 percent humidity.
The foliar half covers powdery mildew, downy mildew, snapdragon rust, Cercospora leaf spot in zinnia with its tan center and purple margin, and angular bacterial leaf spot with a yellow halo. Viruses close it: INSV in snapdragon, TSWV in zinnia, thrips and aphid vectors, no cure, and an Agdia ImmunoStrip that reads like a pregnancy test in five minutes for six to eight dollars.
CHAPTER TIMESTAMPS
0:00 Introduction and Bethany Harris
0:27 Ghimire’s Opening and Background
0:49 Open Field Production and Weather
1:19 High Tunnel Production: Pros and Cons
1:54 Soil, Previous Crop, and Survival Structures
2:29 Raised Beds and Organic Amendments
2:53 Organic Versus Conventional Fungicide Options
3:24 Why Cultural Practice Comes First
4:16 Root and Stem Diseases Versus Foliar Diseases
5:08 Where the Pathogens Come From
6:34 Root and Crown Rot: Reading the Symptoms
7:26 Water Molds, Sporangia, and Zoospores
8:10 Managing Phytophthora: Water and Drainage
9:06 White Mold, Sclerotinia sclerotiorum
10:21 Sclerotia and Five Years in the Soil
11:02 Southern Blight, Agroathelia rolfsii
11:47 Burying Sclerotia and Preventive Fungicides
12:52 Fusarium Stem Rot in Lisianthus
13:36 Botrytis: Gray Mold in Field and Storage
15:16 Botrytis Management: Deadheading and Spacing
15:54 Powdery Mildew and Host Specificity
16:31 FRAC Groups, DMI, QoI, and MilStop
16:54 Downy Mildew
17:28 Leaf Rust in Snapdragon
18:34 Fungal Leaf Spot: Cercospora in Zinnia
19:01 Leaf Spot Management and Irrigation
20:02 Bacterial Leaf Spot: Angular Lesions and Halos
21:10 Virus Diseases and Their Insect Vectors
22:23 ImmunoStrips: Five Minute Field Testing
23:42 Biological Control Products
24:16 Resources: e-GRO and the Pest Management Handbook
25:02 Q&A: Aster Yellows in Coneflower
26:13 Q&A: Botrytis in Tulips, Year to Year
27:46 Q&A: Bacterial Stem Rot and Pectobacterium
QUESTIONS & ANSWERS
Q: Does a high tunnel give me fewer disease problems than the open field?
A: It gives different ones. Ghimire says at 1:19 that a high tunnel reduces airflow and raises humidity and heat stress, which favors several of the pathogens he covers, and he sets that against the field’s exposure to drought, rain, storm, wind, and temperature extremes. What the tunnel buys is irrigation control, an extended season, longer plants and thicker stems. His reason for opening with the comparison is practical: the same crop gets one disease under cover and a different one in the field, and which one depends on the production system.
Q: If I am certified organic, what do I actually have to work with?
A: Biological agents and plant extracts, and he is direct at 3:01 that the options are fewer because chemical fungicides are off the table. The constraint that matters more is timing. A biological product can only be applied preventively, since it has no curative activity, where some chemical fungicides do. That is why he puts cultural practice first for every grower and not only organic ones.
Q: Where does the pathogen come into my operation from?
A: He lists the routes at 5:08. Infested seed, and he names downy mildew and bacterial leaf spot as diseases that ride in on a bad seed lot. Infected plants brought in from outside, which are often asymptomatic at arrival and start showing symptoms later. Contaminated trays, pots, tools, pruning equipment, and benches. Crop residue from previous seasons. Wind-blown and rain-driven spores. His warning is that a field with no visible problem can be building inoculum, and two, three, or four years on you get an epidemic in a bed you never treated.
Q: My plants are yellowing and wilting in patches. How do I tell a root disease from stress?
A: Pull the whole plant, roots included, and look at the root and crown region. He says at 6:34 that this symptom set is confusing precisely because abiotic stress and pathogens both produce it above ground. Below ground a root disease shows reddening or other discoloration, and the root system goes soft and sloughs off. He adds a bad odor as a diagnostic you can use in the field.
Q: Why does overwatering keep coming up with root and crown rot?
A: Because the pathogens are water molds. At 7:26 he points to a video of a pathogen producing sporangia, and inside the sporangia sit zoospores, which are motile. They need water to move. Given standing water they swim freely to a healthy root system and infect it. His management follows from that: water management is the key, poor drainage and lowland spots where water stagnates are where the problem shows up, and incorporating organic material helps the soil and the drainage.
Q: What can I do about white mold and southern blight once the sclerotia are in the ground?
A: Both produce overwintering sclerotia that hold in soil for years, and he cites papers giving five years or more for Sclerotinia. For southern blight at 11:47 he gives a physical answer: dig out the infected plant, then dig the soil about eight centimeters deep, flip it, and bury the sclerotia deep enough that they cannot surface. The pathogen remains but stops being a problem. He is emphatic that infected plants never go on the compost pile, because next year’s compost brings the structure back.
Q: The two diseases sound alike. What separates them?
A: Temperature. White mold (Sclerotinia sclerotiorum) loves cooler temperatures, high humidity, and moisture, and hits cool season crops hardest, showing tan stem lesions and white fuzzy mycelium with black sclerotia. Southern blight (Agroathelia rolfsii, which he notes used to be called Sclerotium rolfsii) wants warm conditions and is at home in Georgia summer, with white radiating hyphae at the crown and sclerotia that look like brown mustard seed. Humidity and moisture requirements are much the same for both.
Q: How do I keep Botrytis out of harvested stems?
A: Storage is the point of failure he names at 13:47. Botrytis is a problem in the standing plant and again after harvest, and improper storage is what turns it into gray mold on the flower. In the field the levers are removing infected plants, removing deadheaded stems and decaying flowers because they are an entry point for the pathogen, good air circulation, and proper spacing so humidity does not accumulate on the leaf surface. Spores go windblown above 85 percent humidity or splash from overhead irrigation and rain.
Q: What fungicide groups did he name for powdery mildew and leaf spot?
A: At 16:31 he names FRAC code 3, DMI fungicides, QoI fungicides with azoxystrobin in group 11, and some group 7 fungicides as effective. He also names MilStop as a biorational product found effective at stopping powdery mildew spread. In the tulip Botrytis answer at 27:19 he adds a two step sequence: a contact fungicide first, naming chlorothalonil and fludioxonil, then a systemic, naming tebuconazole, azoxystrobin, and boscalid.
Q: How do I confirm a virus without sending a sample off?
A: Agdia ImmunoStrips, which he covers at 22:23. Macerate the infected leaf, drop it in the buffer bag with the strip, and read it in about five minutes like a pregnancy test: two lines positive, one line negative. His example shows a zinnia positive for TSWV and negative for INSV. Strips are also available for TMV, CMV, Xanthomonas, Phytophthora, and Rhizoctonia, they can be run in house or carried to the field, and he puts the cost at six to eight dollars per strip.
Q: A grower is seeing aster yellows in coneflower for the first time. What now?
A: Uproot and dispose of the plant safely. At 25:21 he identifies aster yellows as caused by a phytoplasma, which he describes as similar to bacteria, and warns that it is confused with herbicide injury because of the witches’ broom symptom. He says he believes it is transmitted by hoppers of some kind and does not recall which, so the second half of the answer is vector control: keep the infected plant from spreading to healthy ones and apply an insecticide against the vector.
Q: If Botrytis went through my tulips, will it be waiting next spring?
A: Yes, and not necessarily in the soil. At 26:29 he says the pathogen produces conidia in a finger-like arrangement that survive in plant debris rather than requiring soil, and they return the following year depending on what you plant. Same host, same cultivar, and the infection restarts. His plan is early scouting on every plant, then a contact fungicide application at the start of the season if you are replanting the same bed, followed by a systemic depending on how severe the symptoms get.
ADDITIONAL RESOURCES
Named on camera by the speaker:
- e-GRO, which he describes at 24:16 as an online platform covering disease, insects, and environmental, physiological, and nutritional disorders. He says at 24:32 “the link for the website,” pointing at a slide. No web address is spoken on the recording.
- Georgia Pest Management Handbook, which he recommends at 24:46 for its list of fungicides with application amounts, doses, and intervals, and tells growers to refer to it before applying any fungicide.
- Agdia ImmunoStrips, named at 22:23 as the source of the five minute virus and pathogen test kits, at six to eight dollars per strip.
- His contact information, which he puts on screen at 25:01 with the words “here’s my contact information.” No address is spoken on the recording, so it has to come from the speaker or the symposium organizers before publication.
SOURCE NOTES
Full track segment at 28:31.0, standard Stage 2 targets, no light track scaling.
Two open flags, and nothing here rests on either. Block 85 carries [VERIFY: "copseps"] as a descriptor between “sporangia” and “zoospores,” and block 182 carries [VERIFY: "bacia"] as the second downy mildew host after sunflower. The water mold chapter and Q&A pair are written from the sporangia and zoospore content on either side of the first flag, and the downy mildew chapter names sunflower alone. Neither flag is recoverable without a human listen.
Mancozeb is recorded as the speaker said it and no item is built on it. At blocks 216 and 309 he recommends “copper-based fungicides like mancozeb” for bacterial leaf spot and for Pectobacterium stem rot. Mancozeb is an EBDC, not a copper. The Q&A pairs above cover his bacterial management without repeating the pairing, and the Stage 4 session must not build a question on it.
Chapter density is one marker every 57 seconds, 34 across 28:31. Denser than one per 3 to 5 minutes because he moves through eleven named diseases in twenty minutes and the last three and a half minutes carry three separate audience questions. Every marker was checked by script against a block start time in the corrected transcript. The closest pair, 16:31 to 16:54, is 23 seconds apart and clear of YouTube’s 10 second minimum. The last marker is 45 seconds before the end.
Two unspoken on-screen addresses, and this makes four segments running. The e-GRO link at 24:32 and his contact slide at 25:01 are both pointed at and never read. Segment 03 has the UT budgets QR code, segment 04 has Cassity-Duffey’s email, segment 07 has the Atlanta Flower Collective website. Five items now wait on one message to the organizers.
No recorded bio. Bethany Harris introduces Ghimire live at 0:00 and returns at 25:02 to run the audience questions, so this segment has no narrated introduction to split at Stage 5.
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)