Tag Archives: Erik Draper

Managing Difficult Plant Diseases: Numerous host plants

Pictured is a healthy Pelargonium bloom (i.e. florist’s geranium), which will soon senesce. Photo courtesy James Chatfield


By James Chatfield, Joseph Boggs, and Erik Draper

Green industry professionals deal with hundreds of different host plants, different species of maples (Acer) and oaks (Quercus), and different genera from Acer to Zelkova, from Quercus to Xanthophyllum.

Each of these plants has their own set of diseases and other problems. Industry professionals need to remember the multiplicity of host plants creates an extra layer of information for them to keep track of: apple scab does not occur on roses, Dutch elm disease does not occur on oak, black knot does not occur on pears, and sycamore anthracnose does not occur on oak. Each host not only has its own set of horticultural best practices, but also its own set of disease weaknesses.

Infectious plant disease management is challenging, requiring an approach of “the prevention is better than a cure, and in fact is typically necessary.” It also needs careful attention paid to the unique profile of each disease. Nevertheless, it is a key ingredient in good groundskeeping and healthy plant management.

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Managing Difficult Plant Diseases: Pathogens change

Pelargonium blooms infested with Botrytis fall on healthy leaves. The result is Botrytis blight on leaves. Photo courtesy James Chatfield


By James Chatfield, Joseph Boggs, and Erik Draper

The reality is all green industry professionals are proud when they notice genetic resistance in plants to infectious diseases. In fact, sometimes they go to considerable lengths and time with plant-breeding to develop plants to provide both desired arboricultural characteristics and resistance to certain diseases. However, they have to remember nature has its own breeding experiments 24/7. Over time, pathogens mutate, overcome resistance, and parasitize plants that once truly had good genetic resistance relative to a particular disease. However, disease resistance is not necessarily forever, so green industry professionals must keep studying, learning, and adapting.

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Managing Difficult Plant Diseases: Weather is unpredictable

Bacterial fireblight only occurs on certain plants in the rose family, such as Callery pear (pictured), crabapple, mountainash, pyracantha, and cotoneaster. Photo courtesy James Chatfield


By James Chatfield, Joseph Boggs, and Erik Draper

The environmental component of the disease triangle is crucial for plant diseases. Plant pathologists talk about infection periods for specific plant diseases and whether these periods occur depend on such things as the number of hours of leaf wetness and relative humidity. Of course, this in turn is influenced by temperature. All this plays into the overall weather, which can be difficult to predict over the short term, to say nothing of an entire growing season.

Temperature and moisture, for example, play a big role in development of fireblight disease. Bad years for fireblight on callery pear or crabapple usually relate to how warm and wet it is during bloom. The years where there are massive blossom infections—the greatest occurrence is during wet weather during extended periods over 16.7 C (62 F)—are those where fireblight is worse. So, it depends on whether these conditions occurred during the bloom of a particular callery pear or crabapple in a particular part of the province or region.

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Managing Difficult Plant Diseases: Disease control is preventative, not reactive

Apple scab disease on crabapple occurs only on certain crabapple types. Though it does not kill the plant, it may make them unsightly with considerable leaf drop. Photo courtesy James Chatfield


By James Chatfield, Joseph Boggs, and Erik Draper

Since green industry professionals cannot see inoculation and infections occur, they must act proactively with regard to plant diseases. In many cases, this means they need to use fungicide applications to prevent the germinated fungal spore from getting into plant tissue in the first place (though there are systemic fungicides that sometimes help).

A better, more sustainable approach is to employ their knowledge of genetic disease resistance, plant health, and plant stress management to optimal effect. Green industry professionals need to recognize Sargent and Adirondack crabapple have excellent resistance to apple scab, compared to thunderchild and hopa. If they know there is a site with the Verticillium fungus well-established in the soil, which they can know from previous Verticillium wilt diagnoses, then they should not plant highly-susceptible species such as Japanese maples (Acer palmatum).

Selecting the right plant for the right site is the greatest preventive maintenance practice tree professionals can use, not only for infectious disease management, but also for overall plant health. Eastern redbuds planted in open sun on unirrigated sites are more likely to become moisture-stressed and are more likely to have Verticillium wilt problems as well as Botryosphaeria canker disease—the two most common infectious disease problems of Eastern redbuds that cause stem dieback and eventual plant death. Preventing these diseases from the very beginning can occur by not planting redbuds in those sites.

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Managing Difficult Plant Diseases: Inoculation, infection, and symptoms separated in time

Sycamore anthracnose occurs only on sycamores and London plane trees and is favoured by infections during cool, wet weather as new leaves emerge in spring. Photo courtesy James Chatfield


By James Chatfield, Joseph Boggs, and Erik Draper

The inoculum that arrives at the plant via wind, splashing rain, or a vector cannot be seen. Then, a spore germinates and penetrates into plant tissue, a nematode inserts its stylus into the plant, or a vector inserts the pathogen into plant tissue, and the pathogen is still not seen. It spreads through the plant and establishes a host-parasite relationship with plant cells.

Turf professionals have no idea the pathogen is there until symptoms develop on the plant (e.g. leaf blighting and discolouration along the veins of a sycamore due to an infection from the sycamore anthracnose pathogen). Symptoms can develop days, weeks, or sometimes even months or years after inoculation, penetration, and infection. Green industry professionals are effectively in the dark all that time. Among other things, this makes effective timing and use of disease-controlling pesticides such as fungicides difficult. Symptom development and damage to the plant may be inevitable even when the infected plant looks fine.

Read the full article: Managing Difficult Plant Diseases