7/2/2013 - By Matthew Shultz

For September I wanted to take a step back from focusing on just one disease that can be present in production nurseries and greenhouses and instead take a look at all four major types of plant  pathogens and what makes each group unique. The treatment schedule for all these diseases is the same, however, preventative fungicide sprays or drenches, every 14-21 days especially during the spring and early summer months will help keep these pathogens at bay. With chemical EOP quickly approaching, this is a good time to talk with growers about what chemicals should be in their pesticide sheds next year.

The first group I would like to discuss is the Oomycetes meaning “Egg Fungi”, formally known as water mold diseases. Pathogens in this group are some of the most common and most notorious in the world. They can also take on many forms, including branch and canopy dieback, root rots, blights, damping off, and even leaf mildew.

The Great Potato Blight that occurred in Ireland during the mid-1800s was caused by Phytophthora infestans, an Oomycete. There are three diseases in this group: Phytophthora, Pythium and Downy Mildews. Phytophthora generally attacks both herbaceous and woody plants and infects branches (Aerial Phytophthora) or root rots (black dead root systems). Phytophthora is especially prevalent on plants in the Ericaceous family. Pythium usually attacks herbaceous plants in the production setting, but some strains will also colonize some woody species.

Pythium is also known as dampening off (root rot) and blight; it is most prevalent in the greenhouse, especially propagation beds. To inspect for these diseases be sure to look at the plant's root system. Downy Mildews attack the undersides of plant leaves, and brown to red to purple spots will start to appear. These spots over time will start to appear on the upper surface of the leaves, and in extreme cases will defoliate a plant.

All of these pathogens use water to help with their spread, so keeping plants on the drier side will help to slow these diseases down. Avoiding overhead irrigation or watering in the morning will also help to eliminate Downy mildews. If you see free standing water at a nursery, there is a good chance your client has a water mold problem. Infected plants should be removed to avoid further spread. Fungicides such as TitlePhyte, Alliette, Subdue, Hurricane, Protect DF, Stature, Pageant, and Insignia work well against Oomycetes.

The next group I would like to touch on is leaf spot diseases. Leaf spot diseases are much easier to monitor as they only infect a plant’s leaves and stems. This group of plant pathogen is the largest, and about 15 different diseases are common in the production setting. Some of the more common pathogens include Rusts, Powdery Mildews, Entomosporium, Alternaria, Phyllosticta, Cylindocladium, Cercospora, and Collectotrichum.

To monitor for the presence of these diseases, simply inspect the upper leaf surface for the presence of abnormal spots or fungal growth. In severe cases plants can become aesthetically unpleasing and even defoliate. Pathogens in this group are most commonly spread by splashing water from rain or irrigation events, so avoiding overhead irrigation can help to alleviate these fungal problems.

Some pathogens in this group can also cause a bleeding canker to form on branches and trunks of larger plants. Check for a hole in the middle of the canker - this means you have borer issues, not fungal problems. These diseases are easily controlled using products such as Heritage, Protect DF, Pageant, Daconil, Banner Maxx, Eagle, Spectro 90, Strike, coppers and thiophanate methyl. Fungicide sprays should be made to newly expanding leaf tissue on a regular basis. Infected plants should be trimmed back and a fungicide should be applied.

The last two groups I mentioned were examples of fungal pathogens, now I would like to discuss bacterial disease. Bacterial disease can be hard to distinguish from the fungal pathogens, as they take on many of the same symptoms as the fungi, including leaf and stem blights, vascular wilts and root/crown rots. Fire blight and shot hole diseases fall into this category and their effects can be seen on the plant canopy.

Fire blight starts at the tip of a branch and works its way down the stem; leaves slowly turn off color and die. Shot hole infects a plant’s leaves and causes a leaf spot initially which eventually kills the leaf tissue and causes it to fall out…hence the name shot hole. Vascular wilts are most common in herbaceous plants. Crops infected with this type of bacterial pathogen look like they have been under watered; the canopy will wilt even though there is plenty of available water. This will give way to severe necrosis and rotting of the plant’s roots and crown.

There are a couple of easy ways to check for the presence of a bacterial disease. First, for herbaceous plants, pull off a portion of the infected plant’s crown tissue. Smell it….if it is one of the most unforgettable rotten smells, then you probably have bacteria problems. Another easy way to check is to cut off a portion of the infected plant and submerge the fresh cut in a glass of water. Wait a few minutes and check for the presence of the bacteria streaming out of the fresh cut. The pathogen will actually push itself out into the water and can be seen oozing from the plant tissue. This technique works very well on herbaceous plants, woodies might require some magnification. There aren’t a lot of options available when it comes to controlling bacteria, but coppers work very well, as well as Cease (which must be used preventatively).

The last group of pathogens are in a class of their own….the viruses. Traditionally viruses were most common on vegetable crops and some herbaceous plants, but with the onset of the Rose Rosette Virus, these pathogens are redefining how we think about them. Viruses are parasites and will eventually kill a host plant. Most pathogens in this group will cause some type of abnormal growth to occur, so knowing what a healthy plant looks like is very important in diagnosing these  pests.

Infected plants will often have some sort of ring spots or mosaics on their leaves, excessive stunting or strappy thin leaves. There are currently no products available for controlling viruses, however viruses can’t spread on their own, they need a vector by which to spread. Most often insects serve as this vector, so controlling insects such as leafhoppers, thrips, aphids and mites will help to control the spread. If a virus is suspected, samples should be taken and submitted to Agdia in Elkhart Indiana for further testing.

These are the basics when it comes to plant pathogens and how to diagnosis them. Hopefully this will give you some tools to help you rule out some of the common suspects. Call your tech rep with any further questions.

By Matt Shultz, Horticultural Tech Support Specialist and Sales Representative, Harrell’s LLC.