We have been receiving questions about the effect of unseasonably high temperatures on plant diseases this year. Unfortunately, there’s not a lot we can predict based on warmer than usual temperatures in terms of plant diseases. Part of the uncertainty comes from not knowing if these temperatures will continue or what the rainfall pattern will be like. However, if warm temperatures persist and we get moist weather conditions, we might see some diseases show up earlier than in previous years.
Plant pathogens in temperate regions can survive the winter inside insects, plant debris, and soil. These organisms have adapted to survive extreme winter temperatures and wait until the right weather conditions come along to start a new life cycle each spring. So what happens when temperature increases earlier than usual? Well, we have to consider that temperature is just one of the environmental cues to which pathogens respond. But let’s think about the other major cue: moisture. If we have enough rain over the next couple of weeks along with mild temperatures, we might see many diseases show up earlier than normal. This could be the case with some common leaf spot diseases such as apple scab, cedar-apple rust, and anthracnose. Infections can also be more severe, cause early defoliation, and several leaf flushes. If you treat your evergreens against fungal diseases such as Rhizosphaera needlecast or Diplodia tip blight, you can expect some application failures due to a shift in timing. Chemical recommendations are based on development and protection of new needles and since new growth is already starting, applications might have to start as early as mid April or when new growth is about 1-inch long. Remember that a second application must be made when needles are fully expanded. Insects can also play a role in plant disease cycles this year. Disease cycles in which pathogens are overwintered and transmitted by insect vectors will be closely linked to the insect’s activity and behavior. For example, we are already under a high risk period for oak wilt transmission, as nitidulid beetles (one of the main vectors of oak wilt) began their activity 2 to 3 weeks earlier this year. For more information about insects and mild winter see the February 8, 2012 issue of the Horticulture and Home Pest Newsletter. Although there’s a lot we don’t know, if you are concerned about plant diseases, there’s one thing you can do to be better prepared. If you haven’t already, there’s still time to clean up your garden and clear any plant debris from last year. This simple practice will ALWAYS help prevent and reduce plant disease problems. Stay tuned for more plant disease information.Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on April 4, 2012. The information contained within may not be the most current and accurate depending on when it is accessed.