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Posts Tagged ‘plant diseases

Plant Disease: Fungus for your Thoughts

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Brian Aldiss’s novel Hothouse takes place in a world that’s stopped spinning, with one side of planet Earth forever facing the Sun, and in which the existing life forms have either been wiped out by the world’s climate extremes or have adapted in monstrous ways to compete for survival.

There is one life form in this dystopia, however, that persisted less aggressively thanks to its ability to reproduce and thrive in more ways than simple plants: fictional or nonfictional, fungus is ever in our soils, lying dormant and waiting for the opportune moment to perform whatever function its been designed to do — for better or for worse.

Peace Tree Farms owner speaking about automated irrigation systems and commercial sales of basil and other herbs

When I hear fungus, my mind can’t help to think of your typical mushrooms clinging to decaying logs on the forest floor or the vomit-inducing growth in bathroom crevices. In fact, the National Center for Biotechnology Information speculates that around 5.1 million fungal species could currently exist.

Even during our Delaware Valley University Integrated Pest Management class visit to Peace Tree Farms this week, our greenhouse guide emphasized the business’s never-ending war against mildew — powdery or downy — on its commercial crops like basil and rosemary. The greenhouse switched over to LED lighting and maintains very low humidity levels during winter to suppress growth of these fungal mildew diseases.

Back at the University’s own greenhouses, humidity and poor air flow persisted amok, the greenhouse fans failing to aerate the dense vegetable foliage that went unchecked during our unusually balmy days of February and March. Several beds’ yellow zucchini squash suddenly exhibited wet fruit ends which quickly became covered with a black-gray coating. Starting out as whitish stands with balls on the tips (don’t laugh), these progressed to the black-gray strands, which, under magnification, was the coating observed on the wet, rotted zucchini ends.

Choanephora ( Species C. cucurbitarum occurs on cucurbits) is one of the fungi that can lie low for a long period of time until conditions are ideal for germination and reproduction. The fungus is able to survive in field debris through the seasons and, worse yet, can be spread by insects, splashing water, or wind. Once it’s observed by the grower/farmer, it’s already too late.

As for many fungal diseases and diseases in general, cultural control is the best option for managing the pathogen. Crop rotation — for example, planting a non-cucurbit species in an area that was previously planted with a cucurbit — is one way to thwart the fungus’ survival. The obvious control based on our DelVal fail is proper spacing — adequate air movement will allow flowers and leaves to dry, the high humidity on which would allow for a favorable environment for Choanephora.

Unfortunately, the dystopia of our DelVal greenhouse plot is tragically true life, and Choanephora forced us to remove our overgrown zucchini squash plants. Had this been a real-life farm situation, economically, we would have been…

Fungus – ngus + sea + kay to the ED.

In any case, keep alert in your greenhouse or in the field for signs of fungal infection; they are inevitable and, in an Aldiss-esque world, will probably one day control our minds.



Experiencing the World Through Plant Diseases

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The more we know about the world, the better off we are to coexist in it.

Four years ago, I visited Hanamiyama Park — located a few minutes’ drive across the river from downtown Fukushima City in the northern region of the main island of Japan — a privately owned property that has become a popular local tourist spot for cherry blossom viewing. Opened to the public in 1959, the owner/caretaker gave the chance for everyone to come and behold the carefully planted sloping hills flooded with cherry and plum trees of pink, rose, white, cream, and yellow.

It was also my first learning experience that sparked my interest in plant diseases, starting with an ominous but intriguing black tar-like substance stealing the color away from the scenic slopes.

Plant diseases and pests affect everyone, not just in Japan but, say, Africa, where climate change has caused major problems in maize production. With rising temperatures causing drier conditions and thus inviting new pests, Kenya expects a major loss in field production of maize, going from 2.1 million hectares of productive land to 1.2 million. These exacerbating conditions follow East Africa’s prevailing losses due to Maize Lethal Necrosis Disease.

Nearly 14,000 kilometres to the southeast, New Zealand is facing the decimation of its more ornamental myrtle trees, notably the pohutukawa tree (Metrosideros excelsa). The government, it seems, has tried to put a stop to the spread of the rust, but it’s clear the disease has already taken its toll on the country’s beautiful native.

Come back to Pennsylvania, and we’re now dealing with a disease once thought contained in the South. Sclerotinia rot, or white mold, affects over 400 species of crops and weeds and can survive up to 10 years in the soil. Even when tilled under once, it can resurface and generate once another till brings it to the surface again. The pathogen enters through senescing (dying) flowers.

While the disease can cause significant crop loss, efforts are underway to manage it (notice “eradication” isn’t the term here): cover crops such as alfalfa are viable alternative hosts for the disease, but monocots (grasses and cereals) are not and can be used in rotation. Otherwise, if the field cannot be planted with non-susceptible plant species, it must be left fallow for several years. Fungicide treatment is an option, albeit costly and requiring years of repeated treatment. Promoting airflow and laying plastic are other cited methods of reducing infection and transmission (Philip Gruber, Lancaster Farming).

Tar-like mold on Hanamiyama tree

Back at Hanamiyama Park, caretakers were at a loss for what to do about the black pitch — sometimes in globby clumps and most often in smooth, evenly spread patches — that appeared on the bark of so many cherry trees. Standing atop a ladder and shears in hand, one worker motioned towards the cherry trees lining the slope and commented “There’s nothing we can do”.

Other caretakers said although they didn’t know what was causing the problem, it did start from within, decaying the branches fully from the phloem to the bark. At the time, possible culprits included pathogens such as Septobasidium (velvet blight) or, less convincingly, Leptographium.

In our Commercial Vegetable Production class’s vegetable plot at Delaware Valley University, the inability to identify a disease and find a solution is just as frustrating as it must be for the employees of Hanamiyama Park.

A silvery pattern that began in the veins of zucchini squash leaves but quickly progressed to whole-leaf coverage vexed students and university professors alike. I reached out to the masses on Facebook in the “What’s Wrong With My Plant” group, an administrator of which cited — most likely — silverleaf whitefly as the culprit of our chrome-painted veggie leaves. The following photos show zucchini squash leaves progressing from very pretty silver venation to entire

Speckles of tan spots, on the other hand, turned out to be an easier mystery to solve and which is even occurring in my own South Philly residence: thrips are a common greenhouse pest, though its still vexing to know just where on earth how they just appear suddenly during early-stage vegetables’ growth. As well as for marijuana plants. Not that I have first-hand experience or anything.

Aphids on Artemisia annua

Equally arcane in origin were the aphids that happily took my (never-sent) invitation to dine on my fragrant Artemisia annua, or Sweet Annie, plant. Whether in the kitchen of my South Philadelphian rowhouse, a cornfield in Africa, or inside the branches of beautiful-but-doomed Japanese cherry trees, plant diseases are ubiquitously difficult to identify and, due to resistance development, difficult to manage.

In any case, the laddered man at Hanamiyama probably carried the most effective method of stopping spread of disease: his pruning shears. While the destruction of diseased plant parts is often cited as an effective way to inhibit diseases in my DelVal classes, the Integrated Pest Management course offers a way to use several methods to combat diseases: safe fungicides, smart cultural farming practices (e.g., appropriate crop species rotations), diversity in plantings, and more. The more we know about how the world works, from Africa to Pennsylvania, the better equipped we are to adapt to it for our purposes rather than force it to adapt to our presence in it.

Shades of Red: New Trials & the 2017 Philadelphia Flower Show

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Verbena x hybrida Lanai Twister Red
(Image from FleuroSelect)

The California Spring Trials, an annual week-long event featuring the newest and improved additions to the horticulture industry, will take place pretty soon in several locations near Santa Cruz. Sneak peaks are reflecting what’s been passing through the greenhouse recently, and I’ve been seeing quite a few shades of red.

The usual dizzying array of petunias (Petunia Good and Plenty Red) and calibrachoas (Calibrachoa Pomegranate Punch) have come through our cuttings room and more and more salvias (Salvia Grandstand Red Lipstick Pink). Then there’s the proven winners that have been around since the 2013 trials, such as the bold and endlessly blooming geraniums.

Some of my favorites have included the unique red hues and patterns on begonias, the streaked and more subtle scarlet waves on cannas, and the usual eye-catching Verbenas, especially the new Lanai Twister series.

While varying concentrations and cycles of naturally occurring chemicals within these plants are responsible for the colors perceived by different species (bees cannot sense wavelengths in the red spectrum and so they see flowers very differently from humans), some colors and especially variegation are caused by viruses.

Commonly seen are the leaves effected by the tobacco mosaic virus, which produces a mosaic of light and dark colors. Or perhaps you visited the 2017 Philadelphia Flower Show and learned about the virus responsible for the beautiful tulips that were all the craze in the early-mid 1600s. The white streaks in these tulips are simply an absence of functioning chloroplasts (cells responsible for photosynthesis and reflecting the middle range of the visible color spectrum, thus appearing green to us) caused by the tulip breaking virus in cells in development and early growth of the plant’s organs.

The virus, while producing gorgeous specimens that were all the rage for some time in Europe (and absurdly valuable), actually weakened the plant, and so you would’ve seen similar, genetically bred tulips at the Flower Show but not those of Dutch Golden Age fame.

And not all streaks and spots are caused by viruses, but that’s a post for another day. For now, enjoy the photos I didn’t take (all photos taken by Mom Dunlap).