The factors that cause most plant diseases

Grace Trinkoff from Santa Rosa asks: What is it that causes plant diseases to occur some years and not others?

The most basic idea behind a plant disease is that three things have to occur simultaneously. This is called the disease triangle, and needs to include the host, pathogen, and the environment. The host is the plant. Some plants can host any disease, while others only attract specific ones. For instance, Photinia and Escallonia are highly susceptible to fungal leaf spot.

The pathogen is the disease. Diseases are most often caused by bacteria or fungi, which are found just about everywhere. But without the right host and the right environment, diseases cannot do any harm. Pathogens can be very specific and infect only one or a handful of hosts, or they can be very broad and are able to live on just about anything.

The environment includes the weather conditions that are needed for the pathogen to thrive and survive. Fungal leaf spot of Photinia and Escallonia occur in spring because the bacteria do well in wet weather and mild temperatures.

Identifying a disease can be tricky business, but there are lots of resources available to help you determine what diseases are attacking your plants.

Once you know the pathogen, its hosts, and the environment that favors the disease symptoms, you can use the appropriate control methods. For instance, grow disease-resistant cultivars. If you see a tomato seed packet with “VFN” on it, for example, this means the tomatoes are resistant to verticillium wilt (V), fusarium wilt (F) and nematodes (N), all of which are problems for tomatoes. Practice good sanitation by getting rid of infected leaves or pruning plants to increase air circulation. Make sure your soil has good drainage and give plants adequate moisture.

Lastly, we can use sustainable or conventional pesticides, but keep in mind that chemical pesticides can treat one problem only to create another, which then requires another treatment. Avoid chemicals by first using organic and sustainable approaches to handle disease problems. If you’ve done all you can and the disease isn’t going away, then it may be time to take it up a chemical notch.

Tina K. of Santa Rosa asks: Why is it that every year, my day lilies develop these rust colored spots on the leaves? They look unsightly and I want to pull them out.

Day lily rust can be recognized by the presence of raised, yellow pustules on the leaves and flower stalks. These pustules look very much like pollen. The sure test to determine if your plant has day lily rust, and not leaf streak, is to take a white cloth or tissue and pull it over the infected leaf. If the tissue picks up a yellowish-orange powder, then it’s rust.

An infected plant may not show signs of rust immediately. Day lily rust spreads and shows symptoms when temperatures are mild.

Like other rusts, day lily rust is spread by spores carried by the wind, so there’s little you can do if your neighbor’s plants have it. If you have a susceptible plant, your day lilies will eventually be infected. The good news, though, is that day lily rust only affects day lilies — not other plants. Purchase varieties that aren’t susceptible to the disease.

The easiest and quickest way to control day lily rust, though, is to toss the infected plants into the garbage, before it has time to spread.

When it comes to day lily rust, doing nothing is not an option. If this disease is allowed to thrive, the day lilies will not only become unsightly, they will develop fewer and fewer flowers. Day lilies are mainly appreciated for their colorful blooms, but paying close attention to the foliage is key to catching an infection from this disease.

Dana Lozano is a horticultural garden consultant and designer. Gwen Kilchherr is an arborist, garden consultant, and horticulturist. They are partners in a Windsor horticultural consulting and design business, The Garden Doctors. Fax questions to them in care of the Press Democrat at 521-5343.