Spots and Rots and Blights...Oh My! Plant Diseases 101

You might find lions and tigers and bears frightening but farmers and gardeners would disagree. Groundhogs and rabbits and deer can be more destructive. And then there are the fungi. Early blight, late blight and anthracnose. Black spot and mildew and scab. Rusts and rots and wilts. It’s a jungle out there!

Fungi are certainly the most common organisms associated with plant disease. I say “associated” because the pathogen, by itself, does not equal disease. A susceptible host plant and the proper environment are also necessary.

Here’s a timely example. By now, tomato growers are probably noticing brown spots and perhaps yellowing of the lower leaves. On closer inspection, you may see that the “spot” is actually a lesion that has concentric rings of dead tissue. There may be tiny black dots in the dead tissue. As the season progresses more and more foliage is killed. Fruit infections cause soft spots and rots. All of these symptoms describe the common tomato disease called early blight. The disease also affects potato and eggplant. Some of you will be happy to learn that two weeds, horse nettle and black nightshade, are vulnerable, too. Since all of these plants are related, it is not surprising that they are susceptible hosts for the same pathogen. Note also that the early blight pathogen has no effect on asparagus, beans, cucumbers or you. (Sometimes folks ask if it is OK to eat diseased vegetables. It is.). The pathogen has a scientific name Alternaria solani.

OK we have a susceptible plant and a pathogen. The final ingredient needed for disease is the proper environment. This pathogen thrives and reproduces well under warm, moist conditions. The weather in the month of June, 2009 in Southeastern, PA was about perfect for early blight. Especially for tomatoes that were left to sprawl on the ground rather than trained to a support system which favored air movement and leaf drying.

Bingo! Early Blight of Tomato!

Maybe that was more than you wanted to know about early blight but it is useful. You can manage this disease (and all others) by thinking about three key disease ingredients: pathogen, host and environment. Sometimes it is possible and effective to eliminate the pathogen. But it can be equally effective to grow non-susceptible host plants or modify the environment. Disease resistant varieties have been developed for many of your favorite plants. Look for them when buying seed. Learn about the conditions that favor disease. Do what you can to change it. For most fungal diseases that means increasing air movement and reducing leaf wetness.

For a descriptions and gory pictures of common vegetable diseases, type “vegetable disease” into the search box at www.agsci.psu.edu.

For a complete guide to growing tomatoes and info on more tomato diseases go to http://pubs.cas.psu.edu/FreePubs/pdfs/uj230.pdf

Thanks to University of Minnesota Extension for the nice image of early blight shown above.

April Showers Bring May Flowers but… June Monsoons are not a Boon for Most Plants

Just checked a weather web site and it confirms the feeling most of us in Southeastern PA have…soggy. Only three days out of the last eighteen did not have measurable precipitation. Six inches in the last 30 days; this is 160 % of normal. Minimal sunshine. Below normal temperatures.

My farm friends are hurting. Can’t plant. Too wet. You might not be thinking of Halloween yet but farmers are because pumpkins are a 100-day crop that needs to be in the ground now in order to make fruit by this fall. The sweet corn you enjoy in August is planted in June. Perishable crops such as strawberries that are at peak maturity are melting in the field. Weeds thrive in these conditions and it is impossible to cultivate wet soil. Saturated soils and continually wet foliage are ideal for many plant diseases. Ever seen plant wilt because it’s too wet? I have.

Mother Nature can be cruel to folks trying to grow stuff …. gardeners and farmers. As hard as this can be on gardeners, consider the farmer whose income is dependent on favorable weather. Most fruit and vegetable farmers will take a dry year over a wet one, especially if they have access to irrigation.

Any upside to all of this? Well your lawn probably looks great. If you planted trees this spring the watering chore has been minimal. And this can’t last forever. Gardeners and farmers have to be optimists because if it ain’t one thing it’s another when you are playing ball with Mother Nature.

For those growing ornamental plants, Penn State has a very helpful web site describing plant diseases and their management. Check it out.

Poison Ivy

As I drove into our office complex today I noticed a member of the grounds crew cleaning up stems that had been sheared from a privet hedge. Most of it was privet, but I also knew that there was plenty of poison ivy in there, too. I had been observing it for a few weeks, thinking about the challenge of killing poison ivy that is entwined in landscape plants.

I stopped and chatted with him to be sure that he knew what he was dealing with. Sure enough, his arms were blistered up from previous encounters with this weed. We shared remedies for the itch, methods to control the weed and ended up wondering if there was anything positive about poison ivy. I know that bees and other insects collect its nectar and pollen; birds eat the seed. The grounds man thought there might be some therapeutic, whole-body effect from having the rash… what a great attitude!

Poison ivy, Toxicodendron radicans, is a native, perennial, woody vine that is the major cause of allergic dermatitis in the Eastern U.S. according to Weeds of the Northeast, (Cornell University Press). It is very commonly found in Pennsylvania and easy to identify, despite the fact that its leaf characteristics vary somewhat, depending on where it is growing. “Leaflets of three, let it be”, is a saying that begins to draw your attention to key characteristics. The center leaflet extends on a relatively long stalk. The upper surfaces of the leaves are often quite shiny, especially when growing in full sunlight. The leaflets are usually lobed or coarsely toothed. Mature plants climb trees with the aid of aerial rootlets and the older stems become rope-like and quite hairy. But poison ivy is often found running along the ground in the forest understory, and there, the leaves are dull green. Both the newly emerging spring growth and fall foliage are red. Contact with any part of the plant causes a skin rash in most people.

Since poison ivy is perennial, it is not easily controlled by mechanical methods such as mulching, pulling or mowing. In woods and other unmanaged areas, just learn to recognize it and enjoy its beauty in all seasons. If it is growing in places where you or others (unsuspecting kids) are likely to bump into it, you can control it chemically.

Two herbicides, glyphosate (sold a Roundup and many other trade names) and triclopyr (Sold as Brush-B-Gone and others) will kill poison ivy. I think triclopyr is most effective. Glyphosate products will kill or injure any green plant that it contacts. Triclopyr will not injure grasses and this may make it useful in some settings. Both products are applied to the foliage of target weeds. In fact, healthy weed foliage is important for maximum uptake and translocation of the herbicide. Neither product is root absorbed so you can be quite selective by applying the herbicide only where you want it. As long as desirable plant foliage is not contacted it is safe from injury. Read the labels for complete instructions.

So, before the year is over, and once the poison ivy re-grows from the hedge (which it surely will) I hope to help the grounds crew selectively and carefully wipe one of these herbicides onto the poison ivy growing in the privet, avoiding all privet foliage.

Swarm Stories

One of Nature’s most interesting reproductive behaviors is honeybee swarming. Honeybees are very social animals and some experts have even suggested that a colony of honeybees can be more properly thought of as a “super-organism”, collectively, rather than individual bees. Honeybees cannot exist for long on their own. They must belong to a colony. Of course, reproduction of individual bees does occur but always for the purpose of restocking the colony with functional individuals who play a role in the bigger “organism”, the colony.

Reproduction of the colony occurs by swarming. One egg-laying individual (a queen), several thousand workers (sterile females) and a few fat fellows (drones) split off from the established colony, leaving behind a new queen and most of the workers. The swarm is seeking a new home, preferably a high and dry hollow space. Hollow trees, chimneys and voids in walls of buildings are fine.

Swarms always make a brief lay-over after leaving the hive and before arriving at their final destination. This is usually a tree limb, shrub, mailbox, car bumper or fence post. That’s when it gets interesting, even for people who usually don’t give insects a second thought. There is nothing like a 3 pound mass of bees to get folks excited. And the take-off and landing is bizarre… a cyclone of stinging insects, moving with purpose.

Most people who get to see a honeybee swarm arrive or depart feel fortunate to have witnessed it. It’s hard to describe. You kind of have to bee there to appreciate it.

Practical matters… honey bees in a swarm are not inclined to sting. They are most interested in getting settled into a permanent home. Most swarms remain at the lay-over spot less than a day.

So, if you encounter one, just enjoy it and soon they’ll be gone. If they end up staying more than 2 days, find a beekeeper to remove it. In fact, beekeepers are almost always interested in picking up swarms, provided they are reasonably close to the ground (less than 10 feet high). Beekeepers provide good homes to wayward bees. Usually, by the time you contact the beekeeper and he makes his way to the swarm, it has moved on. In any event, swarms are generally harmless and temporary. It doesn’t do much good to try to make them do what you want unless you are a beekeeper or trained exterminator. Certainly spraying them with water or insecticides is not productive.

Swarm season in Pennsylvania lasts from about mid-May until mid-June. Enjoy it while you can. Click on swarm picture above for a close-up look.