November 2015 Plant Health

New Diseases on the Horizon By A.R. Chase

Does it seem like every time you turn around, you are hearing about a new disease, insect or weed? The better our diagnostics are, the more things we find. It is also common that new plant genera or species might be accompanied by new problems. So why do you want to hear bad news? The biggest reason is that if you don’t know about it you cannot take steps to prevent a new disease or at least minimize losses.

If the disease/pathogen is not found in the United States, it may be under a federal quarantine. Quarantines are designed to prevent spread throughout the country. So when you are worried about repercussions of alerting the authorities about a suspected outbreak, remember that stopping a problem early is the only way to keep losses low.

Losses extend from plants that are too badly damaged to sell into costs of scouting, fungicide applications and labor to dump yet more plants. It’s even worse if the new disease spreads from a single infected crop into other crops in your facility. Then, costs of a new disease explode.

Where Do New Diseases Come From?

It is also getting to be common to see “new” diseases originally discovered 100 years ago resurface and become a serious concern for today’s grower, landscaper or gardener. One of the reasons for this is that we are constantly moving crops from propagation in one place to production around the world. The international nature of our industry (and really all of agriculture) makes movement of all pests very common. It is also a fact that every step we take to solve one problem can end up making another one worse.

Better Diagnostics

Our technology allows us to diagnose problems to a very high degree of specificity. Our understanding of what each new wrinkle means is really limited. So while we can figure out that this new thing is really new, it might be relatively unimportant or it might be another serious disease for the ornamental industry. Not all new problems are actually serious since they may result in little if any changes in how we grow the crop. The other side of the diagnostic equation is an expectation that we should be able to be absolutely sure that an unrooted cutting is pathogen-free. All testing is based on a certain population of the fungus, bacterium or virus before a positive test result can occur. It all presupposes that pathogens are evenly and completely distributed in the plant material — this is not true for any disease I know of. A positive test clearly proves something is there. A negative result only means you did not find enough for the procedure to find it.

Newest Disease Outbreaks

Gladiolus rust was found in a few states about 10 years ago. It was originally under a federal quarantine that was recently discontinued due to widespread dissemination.

Impatiens downy mildew was originally described in the United States over 100 years ago but became an issue starting around 2004. It spread rapidly all over the country and remains a problem for growers and landscapers.

Basil downy mildew was originally described from Africa over 100 years ago. It became a “new” disease for the United States about four years ago when it was apparently introduced from offshore.

Phytophthora diseases are on the rise in almost all crops. P. nicotianae is newly reported on phlox from South Carolina. New species are being found and described in Europe (P. pachypleura on aucuba) and many in Korea. In tropical climates, we are seeing outbreaks of aerial blight caused by P. nicotianae on croton and ZZ plant and P. palmivora on pachira.

Other new diseases to be aware of include :

• A new form of Anthracnose on sansevieria

• A new form of Xanthomonas leaf spot on ficus

• Ash dieback (Chalara fraxinea) reported in several parts of the United Kingdom in 2012

• New tospoviruses (like INSV and TSWV) in Florida including groundnut ringspot virus and chlorotic ringspot virus (also reported from China)

• Calibrachoa as a new host of Podosphaera xanthii, which causes powdery mildew on verbena and cucurbits.

What Not to Do

One of the most notable new diseases has been boxblight of boxwood caused by Cylindrocladium pseudonaviculata. This disease was widely reported in Europe and the United Kingdom for almost 10 years before it was reported in the eastern United States. It took a very short time to be found almost everywhere that boxwood are produced in the United States and Canada.

Handling a new disease can be tricky. One large woody producer in the southeast United States did everything they could to make sure the rooted cuttings they obtained were pathogen-free. They kept the cuttings isolated, used a preventative fungicide program and routinely sent samples in for lab testing. On the surface it all sounds good.

After months of this approach and all negative lab tests and no symptoms, they decided they were clear and went into major production mode and stopped the fungicides. After all, why use fungicides if they are not needed? The sad outcome was an enormous outbreak of boxblight and very large monetary losses.

What went wrong? You cannot test a plant for freedom from disease using the methods we currently have. You can only check plants with symptoms to see what is causing them. If you spray preventative fungicides you will mask an infection. To find a latent (hidden) infection, you must not use fungicides or bactericides.

Conclusions

So what can you do? First, stay informed by attending meetings at least. Ignorance is not bliss for very long. Know what to watch out for and be very careful how you procure new plants. It is always a good idea to quarantine new plants, even if you have been growing them for years. If you change sources of cuttings or plugs, you should be especially aware of any unusual symptoms. And always ask for help. Don’t assume anything.

New Diseases on the Horizon