Blight, Rust, Mold, Rot, Slugs, Snails and Earwigs
are all the wages of a wet summer, but the greatest of these is Late Blight. Our resident mycologist has the scientific perspective – and as he is also our tomato maestro, a very heavy heart. Here is his report on the New York garden, with a full explanation of the disease and how it spreads:
LATE BLIGHT –PHYTOPHTHORA INFESTANS– SWEEPS THROUGH THE NORTHEAST
I was in Maine when the word came in: Late Blight was laying waste to tomato fields in the Hudson Valley. Oh, say it ain’t so I pleaded. Leslie and I had been extra cautions this spring in starting from seed, setting out, cultivating, and protecting our tomatoes, fifty or so plants of some twenty or so varieties, mostly open pollinated heirlooms. They were specially grown from selected seed in our own greenhouse and in another in Maine; some heirlooms were painstakingly grafted onto disease resistant rootstock. The spacing was good. The plants were held up by a twine-between-post well-ventilated system, the ground carefully mulched with bright straw over paper, and all the lower branches were removed so as to exclude the transmission of soil borne pathogens. In addition the plants had been treated with Bacillus subtilis (Serenade tm) to protect against fungal infection. Say it ain’t so I prayed as I piled into the car and raced home through a driving rainstorm, thick as I had ever experienced.
But it was so.
Although the infestation seemed to hit our tomatoes later than others around us, the garden plots were thoroughly infected. Some plants were totally gone, black and shriveled; some seemed green and healthy, yet a close inspection of even the healthiest of plants revealed the telltale water-soaked lesions characteristic of Late Blight. Although I had studied Mycology in Grad school with a Phytophthora specialist, I needed a crash refresher course, so I hit the internet. After reading as much as I could about the Blight I realized I had no options but to cut my losses and cut the plants, removing them from the garden, if not my mind and heart.
There’s a thorough summary of advice at the end of this fact sheet from Kentucky State University. If you are also a gardener this will be familiar. If you are not and purchase your tomatoes from the greenmarket, this is why they are so expensive this year. It is a sad story.
The Historical Context:
As you probably know by now Phytophthora infestans is the fungus that was responsible for the Irish Potato Famine of the 1840’s. If you are of Irish extraction it is likely your ancestors came to North America as a result of this blight and the political forces attendant to it. Two million, about 30%, of the population of Ireland, either died or migrated in a five year period following the Blight, They called it An Gorta Mor, The Great Hunger.
In Ireland rain is common, and in 1845 a fungus brought to the Emerald Isle from America found perfect conditions for reproducing in potato fields. That spring the weather was rainy and cool for weeks on end, allowing the blight to spread from leaf to leaf, plant to plant, field to field, town to town in a matter of days. Crops that looked green and lush on Monday lay in rotting heaps by Friday. The speed at which the infection moved was explosive and unprecedented. It caught everyone by surprise, and forever changed the face of both Ireland and the United States.
Fast forward to summer 2009; from Ulster in Ireland to Ulster County in New York. Coming on the heels of an already cool wet June, wonderground.com forecasting system reported that… The combination of cool and wet conditions made July of 2009 the worst July on record in Albany New York. The 9.91 inches of rain which fell during July of 2009 made this the wettest July on record with records dating back to 1826. The average temperature of 68.3 degrees during July of 2009 tied for the 3rd coldest July on record with records dating back to 1820. No other July on record in Albany had a combination of cool and wet conditions of this magnitude.
Conditions for the spread of Phytophthora were perfect.
Add now the pathogen. Plant scientists at Cornell have determined that a few plant distributers were the initial providers of infected plants. These were shipped to Big Box Outlets (Wal-Mart, Lowes, Home Depot, Etc) throughout the northeast in time for spring planting. As these infected plants were placed on the shelves of the stores, they infected others, which in turn infected others, which were brought home to be planted. Loci of infection were begun all across a wide swath of farms and gardens from New Jersey and Pennsylvania to Maine. And then it rained, and rained and rained… The infection was earlier and more widespread than any ever recorded for the Blight in America. Farmers en masse were forced to plow under their tomatoes, the major cash crop of many, as the forces of just-in-time global-inventories came home to roost.
The Mycology: classification
Until recently Phytophthora was classed as a fungus, in Division Mastigomycota of the Kingdom Mycetae (fungus), a division characterized by fungi with motile, swimming spores, called zoospores. A further classification, Oomycota, was based upon the sexual structures which produce the thick-walled resting oospore. Since the cell walls of Phytophthora, however, are made of cellulose, not chitin, and since their energy is stored as glycogen, not starch, Phytophthora has since been reclassified into the new Kingdom Stramenophyla which includes Oomycota and some Algae. Nevertheless, the study of Phytophthora is still treated in mycology courses and control treatments for the Potato Blight are based upon the use of fungicides.
This name change and reclassification is a process frequently met in the study of fungi. Although the names change, the fungus, or in this case what used to be called a fungus, remains unchanged.
Oomycota and Zoospores
The reason Late Blight outbreaks occur in wet cool seasons is due to their zoospores; spores that can swim around like little tadpoles or sperm.
Unlike sperm, however, these spores have two tails, one pointing backwards and one forward. They swim out of the parent zoosporangia and then either keep their tails and swim to a nearby site to infect, or shed their tails and rely upon the wind to carry them to new sites. They can be carried by the wind for considerable distance – up to 50 miles it is thought – and then when they find a suitable host they form a germ tube and enter a cell of the host plant. After infecting the plant, by either method, they may live either within the cells or between them. The variety of mechanisms by which they are spread, infect, and feed are thought to be determined by both the temperature and relative humidity, and give them extraordinary pathological opportunity and strength.
Three or four days after infection, the leaf will develop a characteristic ‘water soaked’ lesion as the pathogenic enzymes break down the plant tissue. A fringe of white fuzz, sporangia, can often be seen at the edge of the spot, often on the underside of the leaf. This produces a crop of asexual spores. Each lesion is thought to produce from 100,000 to 300,000 spores per day. You can explore more of the biology here.
Late Blight is sometimes described as an “obligate parasite” able to live only on certain plants of the Solanaceous family – potato/tomato/eggplant, etc. It is now known however that P. infestans can also infect and be harbored on other plants such as wild nightshades, bittersweetsand petunia.
Other species of Phytophthora zero in on other plants. Beginning in 2000 the closely related P. capsici was seen to cause blight in cucurbits (melons, squash, cucumbers), as well as peppers, lima, soy beans, purslane and other weeds. By 2008 it was found to have infected snap beans on Long Island.
And P. ramorum, which causes Sudden Oak Death, can also affect dozens of other tree and shrub species.
Sexuality in Phytophthora
The genus contains well over fifty different species which have adapted to exploit different host plants. The rapid adaptations within the genus are facilitated by sexual recombination and the consequent reproduction of the new genetic patterns. For this to occur, of course, sexual differentiation is required. In Phytophthora the ‘sexes’ are referred to as A1 (male) and A2 (female). Possibly the only positive aspect of the 2009 infestation is that only one (homothallic) strain is currently in the Northeast – or at least that is the hope.
Both the Solanaceous family of plants and the specialized parasites of Phytophthora are thought to have developed in Mexico. Consequently, and for obvious reasons, both A1 and A2 strains of Phytophthora are present there, allowing for heterothallic sexual unions which both shuffle genes around and create the resistant, resting oospores.
Just as our southern neighbors gave us our tomatoes and potatoes, they also gave us the obligate parasites. The first Potato Blight in the Northeast was recorded in the mid 1800’s a year or two prior to the Irish outbreak and for over a hundred and fifty years there was only one strain which appeared here. Without a sexually produced resting oospore, the disease could not overwinter in the soil. It could only be transferred asexually by zoospore from one living plant to another during the growing season.
More recently, however, newer, more aggressive forms of Phytophthora have come out of Mexico and currently both A1 and A2 strains are found in the US, primarily in southern states.
Complicating the situation is the fact that the sex of homothallic populations can change as a function of the food availability, the ‘males’ forming when food supply is abundant. And, as it turns out, weather conditions which promote an abundant potato harvest are the very same ones which favor a Phytophthora outbreak.
Should compatible heterothallic strains appear in the Northeast the conditions would then allow sexual mating and set the stage for persistent overwintering strains of ever more resistant, dynamically adapting strains of Phytophthora. The oospores of this pathogen would then be ever-present just waiting for the rain, fog, drizzle and dew. The magnitude of this problem sort of reminds me of the advice given for protection from nuclear attack when I was younger. How did it go?… Just crawl under a nearby desk, bend over, place your head between your knees and kiss your ass goodbye! Yikes!
A word about the weather and climate
Long range weather forecasts that I have seen predict August to have a continuation of the pattern set up in June and July.
Weather and climate are different concepts, with the latter referring to the more widespread and long term weather patterns. But this we know about a warming climate: the warmer the earth becomes the more energy available for weather systems. Evaporation and temperature gradients both increase.
The climate on average may change by only a degree or two, but the weather will be characterized by more volatile, more energetic, and more extreme conditions. This summer the rains are here in the Northeast, the drought elsewhere. A subtle shift in the temperature over the south Pacific however, or the lack of snow cover over the northern lands masses, can suddenly cause the jet streams to shift and with it local weather patterns.
In short, a warming world is more dynamic, both as to the weather patterns and the biological communities which adapt to these conditions. Since most rapid change is destabilizing, the task set before us will more and more call for our human institutions to also rapidly adapt.
In the late 1980’s and early 1990’s medical journals, such as The Lancet, devoted entire issues to the threats posed by emerging disease pathogens in the face of climate change. Our collective response seemed to be to bury our head in the sand, to buy more air conditioners, hummers, SUV’s and cartoon commentators, all the while waiting for the excrement to hit the ventilation system. History may well record that it hit this last year and the fallout appears to be everywhere, in my garden, backyard, community as well as yours.
Say it ain’t so, I pleaded, as I raced through the driving rain on the Mass Pike, over flooded rivers and streams in Columbia County, past houses and fields under water, to my wrecked little tomato patch, the radio thick with the news that the Cash for Clunkers program burned through a Billion dollars in only five days, that the recession was receding, that the streams were rising and that the Northeast had been declared an Agricultural Disaster Area.
Say it ain’t so, I pleaded… but it was.