Growing plants need water, and many growers use natural bodies of water to irrigate. Unfortunately, lakes and ponds tend to support other forms of life – life that can harm carefully cultivated crops.
Talking about the proactive control of water molds (and other waterborne pathogens) at the most recent Cultivate event was Bryan Fuhrman, Ph.D., an aquatic scientist with SePRO.
His goal with this management is twofold: To control molds and to optimize the resilience of root systems and microbiomes.
“We want to reduce the load – it’s impossible to eliminate,” Fuhrman said of water mold. “We also want to minimize pathogen introduction from irrigation.”
The pathogens can be tricky. For example, water molds aren’t really fungi. Real problem fungi may be Botrytis, Rhizoctonia, Fusarium and most rusts (pucciniales); problem molds (also called oomycetes or “fake” fungi) are Pythium, Phytophthora, downy mildew, Globisporangium and white blister rust (Albugo candida).
“The biggest commonalities are they all love water and thrive in high moisture,” Fuhrman said. “This can lead to soilborne disease. Periods of dryness are okay – if they stress your plant, they will stress water molds more.”
Waterborne Fungus
These “real” fungi have normal fungus metabolism and have chitin cell walls. That chitin triggers plant threat responses (known as systemic acquired resistance or SAR). The presence of chitin in fungus puts it at a disadvantage.
A waterborne fungus prefers well-oxygenated surface water. It often lives on aquatic plants and algae and “loves biofilms,” according to Fuhrman. It can generally eat either dead or live organic matter and it adapts to its environment to decide on which – more live plants/algae mean more opportunity for parasites – and live plants have more nutrition – but live plants might put up a fight. Dead organic matter doesn’t fight back.
Good news: Most fungicides work on waterborne fungi, so there’s more options for control. The compounds that induce SAR are more likely to be effective too. Minus chemical control, growers can also do what they can to limit the growth of algae/plants/biofilms, especially near irrigation intakes. Also note that the more organic matter your growing media/soil has, the better it can deter parasitic growth.
Water Molds
Fuhrman stressed that water molds are not fungi. They are algae that stole fungal traits.
A major difference is water molds have cellulose cell walls (containing no chitin); plants also have cellulose cell walls, so adaptive responses are delayed. Water molds also have a different metabolism, and that’s why a lot of FRAC groups don’t work on them.
Water molds have a variety of spores and starter structures (mycelia or branches that hold resting cells; oospores for sexual reproduction, with thick outer coatings which are more resilient and can stay dormant; and zoospores for asexual reproduction, which are more mobile but don’t persist as long). “They’re very clever,” Fuhrman said.
These pests prefer stagnant water with low oxygen content and lots of sediment. They produce dormant cells that hang out in that sediment – the dead algae and plants that settle on the bottom stimulate their growth.
Pythium and Globisporangium can eat both dead and live organic matter, “which might explain why Pythium is more difficult to control – it has metabolic flexibility,” Fuhrman said. Phytophthora and downy mildew are purely parasitic, feeding only on live matter.
Fortunately, water mold control is fairly straightforward. Growers should aim to limit algae growth; limit “muck” (organic-rich sediment which is black and fluffy) from building up; and circulate (use an aeration system or pump to keep water moving) their irrigation ponds.
“Remove the training grounds of these soldiers,” Fuhrman directed.
Proactive algaecide can kill water molds. A proactive solution prevents a small bloom of algae from becoming massive. It’s best to use in spring; you can also use it for maintenance (every two to four weeks for the best control). Early season use will kill most live water molds and many dormant water mold structures.
Gray mold (Botrytis cinerea). Photo courtesy of Petr Kapitola, Central Institute for Supervising & Testing in Agriculture, Bugwood.org
Proactive Pond Management
Fuhrman suggested following this simple rule: A body of water that is clean and pristine is less comfortable for pathogens.
Minimize the live and dead plants and algae in your water source (which helps prevent muck accumulation). “This all starts early – it’s tough to keep up with ponds after algae or aquatic plants get started,” he said. “Dead algae and plants in sediment leads to nutrient release and those nutrients fuel the growth of new algae and plants. The less stuff in the pond, the better.”
Algaecides can also reduce muck and water mold presence when used periodically and proactively.
What about the flora around the pond? “Plants around the edges are perfectly fine. A small amount of submerged or floating plants is not a problem. Excessive plants throughout the pond is the problem,” Fuhrman explained.
Growers can control excessive aquatic plants using aquatic herbicides (systemic and early in the season is best). Using dyes can limit the light available for photosynthesis.
Seasonality affects algae growth as well. It’s driven by temperature, light and nutrients. “We know ponds are no one’s main priority, but forming a strategy in winter for control puts you ahead,” Fuhrman said. “Late March to early June is when they usually start growing, so you need to act before that.” Ideally, growers would apply algaecide one month to two weeks before its growth.
As for pond water nutrients, phosphorus (P) is key. More P means more algae. Nitrogen (N) and potassium (K) are not as important for algae growth – but seedlings in the greenhouse require high amounts of P.
A common problem is growers use high-P fertilizer in spring, and then that irrigation runoff goes into their ponds, and then those ponds have high P, which leads to excessive algae growth.
Phosphorus Management
First, Fuhrman said to monitor and adapt your fertilizer plan. Monitor the P concentration in irrigation runoff – meters are available for this. If it’s high (more than 200 ppb), you’re using too much. You can use less soluble P and more slow release P fertilizer (like bone meal).
Next, remove P from the pond itself. This is very effective before the start of algae growth. Absorbent products are available; alum works exceptionally well in less eutrophic (rich in nutrients and low in oxygen) ponds. Fuhrman said alum is cheaper but more difficult to use and it’s best to have an expert apply it.
Dye limits the light algae receives – and it’s an easier approach to manage.
Use a squeeze test. If you can turn your pond sediment into a ball, it’s probably okay. If it’s muckier, it may need chemical control.
Four Simple Steps
There are four ways to proactively reduce pathogen abundance in pond water, per Fuhrman:
- Reduce P in the water
- Aerate or circulate ponds
- Apply dye in early spring
- Use a copper-based algaecide early in the season
He added that when using pond water (or any irrigation system), alternate wet and dry periods – overwatering is always bad.
Fuhrman also reminded growers to not stop using traditional chemistry just because they manage their ponds. “Beneficial fungi are not reliable enough to be the silver bullet. However, you can use them synergistically if you are strategic,” he said. “Fungicides kill all fungi, good and bad. They leave ‘holes’ in the microorganism community. These holes are open after the fungicide is bound with the organic matter or decomposes.”
by Enrico Villamaino
