Speaker: Bikash Ghimire, Ornamental Pathologist, Department of Plant Pathology, University of Georgia
Introduced and moderated by: Bethany Harris
Webinar Date: July 23, 2026
Duration: 28:31
Segment: 08 of 08
Series: Cut Flower Symposium
CEU Categories: [PENDING]
About This Session
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. 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.
The Diseases He Covers, With Conditions and First Response
| Disease | Pathogen as named | Conditions it favors | First response he gives |
|---|---|---|---|
| Root and crown rot | Phytophthora, Pythium | Overwatering, poor drainage, lowland and standing water | Water management, incorporate organic material for drainage, remove the whole plant with roots |
| White mold | Sclerotinia sclerotiorum | Cool temperature, high humidity, moisture, cool season crops | Remove infected plants, keep them off the compost pile |
| Southern blight | Agroathelia rolfsii, formerly Sclerotium rolfsii | Warm, humid, high moisture, Georgia summer | Dig out the plant, turn soil eight centimeters deep, bury the sclerotia |
| Fusarium stem rot and wilt | Fusarium | Not stated; reported heavily in lisianthus | Scout early, remove the wilted plant as soon as it is spotted |
| Botrytis blight, gray mold | Botrytis | Wet, humid, moderate temperature; sporulates above 85 percent humidity | Remove infected plants and deadheaded stems, space for air circulation |
| Powdery mildew | Not named to species; host specific | Warmer, humid but dry conditions | Remove infected plants early, fungicides, MilStop as a biorational |
| Downy mildew | Not named to species | Not stated; systemic through the vascular region | Not stated beyond general management |
| Leaf rust | Not named to species | Not stated; spores travel kilometers on wind | Early scouting, remove infected plants, preventive fungicide |
| Fungal leaf spot | Cercospora in zinnia | Leaf wetness, high humidity, poor air circulation | Remove debris and dead stems, keep foliage dry, stop overhead irrigation |
| Bacterial leaf spot | Not named to species | Leaf wetness on the foliage | Water management, remove infected plants, do not save seed from them |
| Virus diseases | INSV, TSWV | Insect vectors, grafting, propagation | Uproot and dispose, control the vector, do not propagate infected stock |
Every cell comes from the recording. “Not stated” marks a gap in the presentation rather than an omission from this page.
Chapters (Video 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 and 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.
Biological Products He Lists
At 23:42 he shows a list of biological controls labeled for the edible greenhouse and says it can be applied to ornamentals. He is candid that his own testing is in turfgrass and that the ornamental work is ahead of him rather than behind him.
- Bacillus products, effective against a range of foliar diseases and root and crown rot
- Trichoderma products, for Fusarium, Pythium, Phytophthora, and root and crown rot
- Neem oil
- Pseudomonas
- Plant extracts
Resources Named on Camera
- e-GRO, an online platform he describes at 24:16 as covering disease, insects, and environmental, physiological, and nutritional disorders. He says “the link for the website” while pointing at a slide. No 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 consult before applying any fungicide.
- Agdia, the source of the ImmunoStrip test kits at six to eight dollars each.
- His contact information, put on screen at 25:01. No address is spoken on the recording.
Publication Notes
Two things must be supplied by hand before this page goes live. The e-GRO web address and Ghimire’s contact information are both on screen and neither is read aloud. Nothing in the recording recovers either, and no audio sweep will help. This is now the fourth segment in the symposium with an unspoken on-screen address, after segment 03’s QR code, segment 04’s email, and segment 07’s website.
The session title is built from the delivered content. He was moved from the 11:00 slot and delivered the 9:30 agenda title, so the agenda line and the delivered talk do not match. Publishing under the agenda title would send growers to the wrong recording.
Mancozeb is not a copper fungicide, and he calls it one twice. At 20:35 for bacterial leaf spot and again at 27:58 for Pectobacterium stem rot he recommends “copper-based fungicides like mancozeb.” The transcript records what he said, this page reports his management without repeating the pairing, and any future course activity built from this segment must not rest on it. A note to the speaker before publication would be the clean fix.
Two words in the transcript are unresolved and neither appears on this page. Block 85 carries a one word descriptor between “sporangia” and “zoospores” that no transcription pass recovered, and block 182 carries the second downy mildew host after sunflower, whose first syllable all three passes drop. The water mold section and the downy mildew row are written from the surrounding text.
Downy mildew is thin on purpose. He gives symptoms, the underside sporulation, systemic movement through the vascular region, and one host he names clearly. He gives no management step specific to it, so the table says so rather than borrowing one from the leaf spot section.
Scaling Note
Target. Stage 3 derives from the Stage 2 package. Segment 04’s website version ran 15.5 KB against a 15.0 KB archive summary, a ratio of 1.03. This archive summary is 14.8 KB, so the proportional target was about 15.3 KB.
Delivered. 17.7 KB, a ratio of 1.19, over target by about 2.4 KB. The overage is the disease table, which runs eleven rows of four columns and costs roughly 2.5 KB on its own. It is the only part of this file a grower with a sick plant will actually use, so it stays and the ratio moves.
What was added rather than condensed. The disease table and the biological products list have no counterpart in the Stage 2 package. The table reorganizes content spread across eleven chapters and six Q&A pairs into the form a grower with a sick plant wants, which is what the website version is for. Its “not stated” cells are honest gaps in the presentation, and marking them is what keeps the table from reading as a complete reference it is not.
Processed for UGA Center for Urban Agriculture / Cut Flower Symposium Archives
Source: Stage 2 Archive Package, GTBOP_Archive_Summary_2026-07-23_GhimireDiseaseManagement.md