Phytophthora in Disease-suppressive Soils

Even the scourge that caused the Irish potato famine can be stifled — through microbiology and mineral nutrition.

In Greek, phytón means “plant” and phthorá means “destruction.” “Plant destruction” sums up a Phytophthora infection quite well. Outbreaks occur primarily in wet, warm weather, and more readily on compacted soils than on well drained. Mainstream agriculture has very limited management options, offering these standard suggestions:

• Prevent it before it starts. (Okay … how?)
• Sterilize equipment after working soil with any history of Phytophthora. (Sure, because sterile and farming go hand in hand, and we all have the facilities to do this…)
• Correct water management and drainage. (Makes sense — because Phytophthora spreads in water, and wet roots are not healthy roots. But fields can still flood, and spores blow in from elsewhere.)
• Crop rotation. (Don’t most published articles say Phytophthora spores survive ten or more years in soil and that no rotation schedule has yet proven effective?)
• Frequent preventive contact fungicide applications with multiple modes of action. (How many years do we repeat these applications before acknowledging that they have not changed Phytophthora pressure?)
• Soil sterilization. (Thanks but no thanks. This “nuclear option” kills soil life far beyond the target organism and turns soil to dirt. Plus it’s difficult, expensive, impractical for most growers, and Phytophthora is known to thrive when introduced to previously sterilized soils.)
• Variety selection. (The best of the above options. But Phytophthora resistance or tolerance is not available for most crops and usually covers only one or two races of the disease.)

The spring and early summer of 2016 was very wet in much of the Midwest, with deluged fields, delayed planting and cultivation, soil compaction and missed applications. The rain brought problems, and one of the more challenging was the scourge Phytophthora.

I was working with a grower in Kentucky at the time and we had a powerful experience showing the value of healthy, regenerated soil. The farm grows melon crops and mixed produce, along with grain acres, and has used a strong nutrition program for several years. The grower routinely uses trace minerals, in-season drip and foliar applications, and cover crops, and has worked to balance bulk soil ratios. The real backbone of the farm, though, is biology. Applications of beneficial microbes and a microbial support package are given to the soil every fall. Cold-processed liquid crab/fish with microbes and humic acids are added on more challenged soils. All of the seeds are treated with a dry seed inoculant containing mycorrhizae and bacteria, along with a liquid enzyme enhancer to ensure optimum conditions for the inoculants.

We had a field walk scheduled at the farm, although we nearly canceled it as the very wet weather had made cultivation difficult — and no one wants to show off a weedy field.

But there was still great interest in the walk. Growers had questions about Phytophthora specifically, as there had been devastating losses in the neighborhood, and this farm had zero Phytophthora problems. The grower harvested 100 high-quality bins of melons per acre, despite experiencing the same spring flooding conditions and planting the same varieties as neighboring farms. The other farmers wanted to know why.

What Is Phytophthora? 

Phytophthora looks like a fungus, behaves like a fungus and is conventionally treated with fungicides. So, like many farmers and crop advisors, I used to think it was a fungal disease. Not so. 

Phytophthora is in an altogether different, newly defined kingdom: Chromista. This kingdom also includes brown algaes, like kelp, and diatoms (from which diatomaceous earth is produced). Within this kingdom, Phytophthora is of a genus of organisms called oomycetes (oh-uh-my-SEED-ees), or water molds. There are plenty of important physiological and reproductive differences to distinguish oomycetes from true fungi, but for those of us without microbiology backgrounds, one critical thing to know is this: oomycetes cell walls are primarily cellulose, just like plants, whereas fungal cell walls are made primarily of chitin.

Also, oomycetes’ nutritional needs are very similar to those of proper plants, but since they lack the ability to photosynthesize, they feed on the tissue of living plants, gathering nutrients and energy with structures called hyphae. This hyphae feeding activity, and the fact that they can reproduce with spores, is why they were classified as fungi until recently.

Physiological similarities to algae explain the diet and water-dependence of Phytophthora; differences from fungi explain why so many fungicides are either ineffective or just temporary suppressants. Most fungicides act on chitin-based organisms and are formulated not to damage cellulose-based plants (though some certainly do). Phytophthora is largely unaffected by this mode of action. Most pathogenic fungi are decomposers, and in healthy soil they assist in digesting weak/dead tissue and crop residues into stable humus. Phytophthora has a minimal decomposing role; it simply feeds when it can on living plants, destroys indiscriminately, reproduces, and moves on, leaving true fungi to do the cleanup.

One Phytophthora, Many Names 

With at least a hundred known species, numerous races of each, and hundreds more predicted to be discovered, Phytophthora is abundant and wily. Some strain exists to attack nearly every plant type — be it crop, wild, annual, perennial, woody or not — and it can attack every plant part from root to fruit.

Red stele in strawberries, late blight in tomatoes, black shank in tobacco, root rot in soybeans and blueberries, various colors and sizes of leaf spot, blights, crown, root or fruit rots, stem lesions and sudden wilts all have the same causative agent. The Irish potato famine was caused by Phyt. infestans, and various infestans strains are still major issues on potato, tomato, pepper and eggplant. Phyt. capsici is the usual suspect on all cucurbits, but it also infects solanaceas. Phyt. nicotianae infects onions, Phyt. sojae the same to soybeans, etc. — not to mention ornamental and wild plants. Damping-off of seedlings is often caused by a Phytophthora … but it can also be caused by Pythium. So don’t jump to conclusions, but you’d have a decent chance of being right if you guessed that a suddenly devastated crop has succumbed to some kind of Phytophthora.

How Did It Get Here? 

Phytophthora is native to most soils globally. It survives up to a decade as resting spores, resistant to drought and freezing, waiting for the right conditions. In the short term, it resides as mycelium on undigested plant debris. 

Disease development starts when rain pools up on soil. After several hours of saturation, zoospores activate and literally swim, hunting for a host. Further mobility is achieved by flowing across slopes, along waterways, in wind-driven rain, on wheels, equipment, boots, hooves and paws. 

There is also a phase of sporulation whereby millions of oospores release aboveground and spread anywhere the breeze will blow. Think about hurricane rains and you see why this is a global phenomenon.

Phytophthora reproduces on an utterly massive scale, both sexually and asexually. Thus, new genetic potentials are constantly bred anew, and copies of successful races are preserved. 

In short, it is here, there, and everywhere. It’s already native to where you are. It’s all-natural, highly adaptable, and loves to travel. We’re not getting rid of it. Be skeptical of anyone who tells you otherwise. 

In this Kentucky field, there was no doubt the inoculum was present and conditions were right. It’s not a matter of whether the organism was present but of what else was there.

Real Solutions

Regular readers of this magazine have heard this before, and it’s true as ever: the best disease prevention is through microbiology and mineral nutrition. Phytophthora symptoms can be successfully and reliably inhibited by common soil fungi and bacteria. This primary line of defense is what is lacking when crops are overtaken. 

Phytophthora mycelium grow quite well in sterilized soil with no microbial competition but are strongly inhibited in diverse, living soils. Why do microbes seek and destroy Phytophthora? It could partly be that other microbes use Phytophthora as a food source, but there is a far more important story here. 

Disease-suppressive soils contain microorganisms that destroy pathogens, such as Phytophthora, that plants are powerless against. After all, if the plant dies, the microbial community that depends on that plant suffers — so it is very much in their interest to promote the sustenance of the whole. 

Allow me to anthropomorphize. Imagine you are a bacterium. You are content, fed and employed, raising a very, very quickly growing family in a new neighborhood on the bustling outer root hairs of your city, “Watermelon Plant.” The plant is mayor, contractor, refinery, factory, bank, grocery, water company, pharmacy and internet provider, all rolled into one. You and the trillions of other diverse residents are all housed and employed by this Watermelon Plant. You’ve invested your life here and have harvested the materials to build it. It’s all you want or need.

One afternoon at work, just sucking on some sugary sap-soda and mining manganese, you come across something strange. Ooo … oh, uh … oospore! Maybe you’ve never actually seen one before, but the description fits. Instinctually, your gut (more or less your entire body) churns uneasily. Recoiling, you call out for help, but no one is near.

It looks harmless, just sitting there waiting, but these things are dangerous! Ruthless interlopers, eager and able to take down a whole city’s infrastructure in a flash! The work of many generations may come to naught if this little gremlin is allowed to get wet. It is the defining moment of your little bacterial life. Into the fray! Destroy it! Be a hero! Then you can die happily of old age (in a day or two), knowing this story will pass to your thousands of grandchildren, and the city can go on supporting its citizens. 

Okay — this cartoon is meant to pound in the point that in the life-and-death struggles of the rhizosphere, organisms can play multiple roles; they have many subtle partnerships, pay-offs and competitions.

Developing Disease-suppressive Soil

This is exactly what is meant by disease-suppressive soil. The makeup of the soil actively works in plants’ favor: hunting, policing and taking action. Beneficials living outside the roots can contact pathogens before they reach actual plant tissue. The more diverse and densely populated the beneficials, and the better they are supplied with energy from strongly photosynthesizing plants, the more territory they can mine-sweep.

Microbial benefits thus go beyond merely munching away to supply fertility. Much remains to be learned about what balances of organisms create the most disease-suppressive soils, strains that excel in specific environments, and how farmers can best guide the process.

With so many variables, it seems daunting. The research work being done in labs and fields across the world must be celebrated and furthered. Much of agriculture is just beginning to see the potential. 

Meanwhile, don’t let the perfect be the enemy of the good! Make progress now. Biological inoculants introduce diverse and healthy microbiology when it is absent. Carbohydrates for microbes along with the right trace minerals support that diverse and healthy microbiology, both native and inoculated. The results are proven and have been replicated on thousands of acres.

The Kentucky grower used proven and effective techniques and products to make a thriving soil ecosystem. We didn’t hold Phytophthora at bay. But some combination of soil microbes did. And they paid dividends.

Nathan Harman is a once and future farmer, a father, and a consultant for Advancing Eco Agriculture. He lives in southern Indiana and works with innovative fruit, vegetable and specialty-crop farmers across the country.