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

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.

Read the full article: Managing Difficult Plant Diseases

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

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

Rose black spot disease occurs only on genetically susceptible roses when there are periods of leaf wetness long enough for infection to take place. Photo courtesy James Chatfield

Inoculum is the structure or part of the pathogen that initiates disease. Microscopic, it can take the form of:
● spores or threadlike mycelia of fungi;
● bacterial cells;
● parasitic eelworms known as plant parasitic nematodes; or
● submicroscopic particles of viruses or phytoplasmas—so small even regular light microscopes cannot detect them.

One of the reasons infectious plant diseases were mysterious for so long, and why disease management is complicated, is the inoculum of the pathogen is invisible to the naked eye when it arrives at the plant, where it then penetrates and infects the plant tissue. Green industry professionals consider insects and mites to be small and hard to see, but they are gargantuan compared to virulent plant pathogens that cause infectious plant disease, along with the other two components of the disease triangle—‘the susceptible host’ and the ‘environment conducive to disease.’

Read the full article: Managing Difficult Plant Diseases

Even though a florist’s geranium may be healthy, bloom tissue does senesce and die, providing just the sort of plant tissue susceptible to the Botrytis grey mould fungus. Photo courtesy James Chatfield

Plant pathogens—such as certain fungi, bacteria, phytoplasmas, nematodes, and viruses—by definition are parasites, infecting living plant cells. However, there is quite a range from obligate parasites (which only survive on living cells) to opportunistic pathogens (which mostly live on already dead cells as saprophytes, but also make a living as parasites on dying or stressed plant cells).

A good example of a pathogen thriving both as saprophyte and parasite is the Botrytis grey mould fungus, which infects many plants from herbaceous geraniums to woody plants in propagation situations. Imagine a world where a perfectly healthy geranium plant has a part that is dying, such as flower blossoms that senesce and die with age. As this living blossom tissue dies, Botrytis colonizes it by feeding on these dead and dying cells. The pathogen rapidly multiplies, especially in cool and moist conditions, and then those infested geranium florets fall on healthy leaf tissue in great numbers with moisture, blocking the sun where they fall. Those healthy leaves then develop Botrytis blight.

Read the full article: Managing Difficult Plant Diseases