A Guide to Disease Management of Tropicals and Houseplants

Tropical and foliage plants thrive in warm, moist, and low-light environments – the very same conditions that invite fungal and bacterial diseases. We spoke with two expert growers who shared their top concerns when it comes to disease in these crops, and how they identify and manage problems before they spread.

Spoiler alert: It all comes down to scouting, roguing, and managing irrigation.

Start with Scouting

“Scouting is the number one way you’re going to prevent disease in your greenhouse or catch it right when it begins,” says Steven Bryant, Director of Growing at The Plant Company.

Trae Crocker, Plant Protection Coach at Monrovia, adds that where you scout matters just as much as how often. “Your biggest problems are going to be in your corners,” he says. “That’s where you’ll find the worst airflow and water overlap. Those are your outliers, and that’s where disease pressure often starts.”

Crocker emphasizes that disease outbreaks usually don’t stem from a single issue. “Usually, it’s not one factor that causes the disease to pop up. It’s a handful of issues, and everything just comes to fruition at once.”

Grey Mold

Among the many disease challenges in foliage and tropical plant production, Botrytis cinerea (grey mold) is by far one of the most frequently encountered, but it’s not the most severe (Check out this eGRO Alert for additional information on scouting for Botrytis).

It thrives in the very conditions these plants prefer – warm, humid environments with extended leaf wetness. Most bedding and floriculture crops are vulnerable to infection, and to complicate matters, fungicide resistance has been documented, making careful rotation of modes of action more important than ever.

“Botrytis is a constant issue, especially when foliage stays wet for too long,” says Bryant. “The gray mold that forms, called sporation, is indicative of botrytis. It has a unique look. You’ll often see it on the soil surface and on the lowest leaves first.”

The best line of defense starts with cultural controls. “It’s all about airflow,” says Crocker. “When you’re talking about fungal and bacterial diseases, you need proper plant spacing and good dry down. We see a lot of the major disease outbreaks trace back to cultural problems.”

“Make sure that you actually have a dry down cycle, don’t let foliage stay wet for more than three hours,” says Bryant. At The Plant Company, that means cutting off irrigation by 2:00 p.m. “Light levels in the greenhouse start dropping around five or six o’clock in the evening,” he explains. “By stopping at two, we give the foliage enough time to dry before humidity sets in.”

Root, Crown, and Stem Rot

Rhizoctonia solani is a crown rot pathogen that spreads across the surface of growing media and infects plants at the base of the stem. It moves easily through irrigation water, on contaminated tools, or via incoming infected plant material. The disease thrives in warm temperatures and moderate soil moisture that many tropical and foliage plants prefer.

Early signs of Rhizoctonia crown rot can be subtle and are often mistaken for drought stress, but there are some key differences. “Look for leaves that are starting to droop from a localized infestation in part of the crown,” says Bryant. “Overall, the plant may still look healthy, but some of the lower leaves might be prematurely desiccated or dropping. You may see signs of stem rot, or a plant leaning to one side – usually affecting just one stem or section. That’s when we start popping them out of the pot.”

When inspecting the crown, you may notice lesions, mushy spots, or both. In advanced stages, cobweb-like fungal growth may appear on lower leaves near the base. To confirm, Bryant recommends destructively scouting the plant by removing soil and carefully examining the crown for tissue damage or decay. Controlling Rhizoctonia starts with cultural practices: maintaining consistent airflow, spacing plants properly, and closely managing soil moisture. Bryant explains that they are quick to act, keeping the problem local. “Roguing that crop, getting the diseased material out of there, and then coming back with a preventative fungicide drench to make sure we’re not spreading the issue is how we manage it.”

Bacterial Leaf Spot

Bacterial leaf spots are a widespread concern in tropicals and houseplants, often caused by pathogens such as Pseudomonas, Xanthomonas, and Acidovorax. Symptoms typically appear as yellow or brown spots or irregular brown lesions on foliage. “Bacterial leaf spot in Philodendron is one of the largest issues that we deal with,” says Bryant.

Bacterial leaf spot can spread with water splashed by overhead irrigation, or by handling infected plants when they’re wet. It can also be introduced through liners, propagation material, and seeds.

“​​A lot of problems with leaf spots can be dependent on the irrigation quality. Not only the quality of the water, but the experience level of the person watering those plants,” says Crocker. He emphasizes the importance of allowing plants to dry down between irrigation cycles, particularly in challenging climates. “You really want to prevent as much leaf wetness as possible. We like to avoid watering later in the day, which can be a lot harder in a hot, humid climate. It seems like everything is just a little wet all the time, especially in the summer when we’re trying to break up these irrigation times just to cool the plants off.”

Bryant echoes the importance of cultural controls. “The biggest way we try to control bacterial leaf spot is by watering only in the morning and keeping that foliage as dry as possible. We use flood irrigation as much as possible and minimize overhead irrigation.”

It’s All About Cultural Controls

“For us, cultural control for disease is the number one thing,” says Bryant. “We actually don’t deal with a lot of disease issues because of all the facility investments we’ve made.”

To prevent pathogens from taking hold, The Plant Company pretreats all its water with peroxide, including the water stored in silos. This step keeps the water source clean and helps prevent biofilms from building up in pipes and nozzles. Additionally, an ultrafiltration system cleans all water after it is recirculated from the greenhouse before it returns to the irrigation silos, removing any fungus, bacteria, or viruses and preventing them from colonizing the irrigation system.

Inspecting plant material as it comes into the greenhouse is just as important as managing what is already growing. The adage about an ounce of prevention rings especially true here. “You can’t control what you’re unaware of,” Bryant says. “Make sure you know what’s going on with your crop, what’s going on with your production facility, and know what’s entering your production facility.”

“I’ll throw away one plant to save 1,000 every day of the week,” says Crocker. “Removing the source of inoculum is huge, because it’s likely a waterborne disease that can drain and infect each pot down the line.” He adds that their team often rogues suspect plants before a definitive diagnosis is made. In some cases, they’ll follow up with a prophylactic fungicide drench to slow the spread while confirming the issue – especially if it resembles one of their more common disease pressures.

“For disease control, it really comes down to staying on top of things – consistent scouting, roguing any suspect plant material, and maintaining clean water sources,” says Bryant. “On top of that, having a solid preventative chemical program to support your cultural practices makes all the difference.”