Extension News

Weather and Plant Diseases -- What’s the Connection?

Fire Blight

Note to media editors: Garden Column for the week of June 3, 2005


By Paula Flynn
Extension Plant Pathologist
Iowa State University

Frustration has been common among home gardeners this spring. Freezing temperatures caused the tender young foliage of many plants to turn black and wilt. In a few cases, such as with tomato seedlings, plants were killed. Fortunately, most plants are beginning to recover,  putting out new green leaves. Waiting for these healthy leaves to emerge can be an exercise in patience.  It’s easy to focus on the crispy brown and black leaves that detract from the plant’s attractiveness.

Gardeners also were faced with waterlogged soils this spring.  Strings of cloudy days delayed the drying of the soil, depriving plant roots of life-sustaining oxygen. The lack of oxygen can cause roots to die and plants to wilt. This damage can appear suddenly or show up more slowly, as is usually the case with large plants such as trees. Poor root health can be a chronic problem in areas with poor drainage. Plants often grow poorly and have leaves that are abnormally yellow. These areas usually require renovation. Also, one needs to be careful in selection of tolerant plants for use in areas prone to water logging. 

Clearly, cold temperatures and excess moisture directly damage plants. Plant pathologists refer to these problems as abiotic diseases, meaning that they are caused by nonliving, or abiotic, factors. Depending on the year, there are many other abiotic diseases that are problematic for plants. The common feature of these diseases is that they are caused by too little or too much of something needed for healthy plant growth. Familiar examples include drought, flooding, high temperatures, low temperatures, inadequate light, sunscald, pollution or nutrient deficiencies. It would be difficult to recall a year when gardeners did not have to contend with at least one of the environmental challenges.

Plants have plenty of adversity to face with just the direct effects of weather. Unfortunately, weather conditions can also influence biotic diseases, those problems caused by living organisms such as fungi and bacteria. For example, the weather this spring has favored the development of certain diseases. Fire blight has been observed on apple, crabapple and pear. The bacterium that causes this disease turns new shoots black, appearing as if they have been scorched by fire.  Windblown rain in the spring helps to move the fire blight bacteria to the newly emerging plant tissue. Cool and wet spring conditions have also favored leaf-spot diseases caused by fungi on various landscape plants, such as shade trees, perennials and turfgrass.  When soils stay cool and wet, troublesome soil-inhabiting fungi can infect plants roots and cause root problems.

On the other end of the moisture spectrum, some fungi tend to infect plants that are stressed by dry conditions. Canker diseases, caused by fungi that encircle and kill branches, tend to be problematic on older, drought-stressed plants. Sometimes fungi infect leaf tissue that has been damaged by cold temperatures or intense sunlight. Many different interactions between the weather and disease-causing organisms can take place.

Although you cannot change the weather, there are some techniques to fool Mother Nature -- at least a little. When planting, space the plants to allow good air flow, so the leaves dry quickly.  Most leaf-spot fungi need the leaves to be wet before they can invade and cause damage. Proper pruning can help improve airflow and sunlight penetration to leaves. Since lack of water to the roots can be damaging, water plants if possible when extended dry conditions occur.  Plants that are growing vigorously are better able to withstand abiotic and biotic diseases. So try to select plants that are tolerant of your site conditions. For example, a bald cypress tree is a better choice for a wet site than a white pine.   

Plant enthusiasts sometimes face unpredictable and extreme Iowa weather, a challenge that is part of gardening.  However, with foresight when purchasing, proper care, and flexible expectations, gardeners can minimize disease occurrences and succeed in growing a wide variety of plants.



Contacts :

Paula Flynn, Plant Pathology, (515) 294-3494, pflynn@iastate.edu

Jean McGuire, Continuing Education and Communication Services, (515) 294-7033, jmcguire@iastate.edu

Fire blight (high resolution)