Plant Pests and Pathogens Evolve Mechanisms to Survive Freezing Temperatures

By Kevin Korus

It is a common question that I receive each year. Will freezing temperatures during the winter take care of the plant pests and pathogens in my garden this summer? Unfortunately, the answer is mostly ‘no’. Although some insects and microorganisms that are exposed to the elements will die, most of these pests have adaptations that allow them to survive freezing temperatures.

Some insects simply migrate to warmer temperatures and move back when weather permits. Those insects that cannot migrate will seek microclimates which are warmer and provide a bit of protection. Plant debris is a common cover for both insects and plant pathogens to overwinter in or under. Beetle larvae can burrow deep into the soil to escape the freeze and other insects are able to survive in dormant life stages like eggs and pupae.

Most microbes will have dormant phases or create survival structures that help them survive adverse conditions. Some plant pathogenic fungi create a unique survival structure called a sclerotium, which is a hardened black structure that is suberized to prevent moisture loss and melanized to protect against UV degradation. Sclerotia can survive freezing temperatures as well. In the Spring, when the temperature rises and moisture is abundant, the sclerotia germinate to release infections fungal structures. Similar to the way insects migrate, some fungal diseases are blown in from warmer climates each year and do not survive our winter season. Some species of rust fungi are known for this. Puccinia polysora, for example, causes southern rust of corn and its infections spores are blown up form South and Central America each Spring and Summer.

Another environmental factor that determines the survivability of microorganisms is moisture. It is much easier for microbes to survive in dry freezing weather than in wet freezing weather. When water freezes it expands and bursts open cells. If you are a single celled organism, like a bacterium, this is a problem. Freezing water also creates crystals that pierce and kill living cells (Figure 1.).  Scientists take advantage of this fact to help with long-term storage of microorganisms including bacteria. Lyophilization is a storage technique that involves completely drying out an organism and then freezing to and storing it at -80° C (-112° F). Organisms that are brought back to temperature and rehydrated will begin to grow.

Figure 1. Ice crystals on a leaf

Plant pathogenic nematodes (microscopic roundworms) have a couple of different ways to survive freezing temperatures. The first is to simply move deeper into the soil profile to avoid freezing. The second method is to undergo anhydrobiosis, a process in which the organism releases all of its water and enters a state of ametabolism. Under anhydrobiosis, nematodes can survive freezing temperatures and/or periods of dry weather.

In conclusion, freezing temperatures may help reduce the amount of insect pests and plant pathogens in the environment but do not ultimately eliminate them. If you have a perennial problem with an insect pest or plant pathogen, contact your local Extension Office for more information on how to properly manage your unique situation.