Matt Karst, Rainbow Treecare Scientific Advancements

As an arborist it can be hard to solve every problem we are faced with. We are slaves to nature and her timeline, and this does not always work in our favor. The majority of the time we are called in when it is either too late, or when things have escalated to an alarming level. This can make things tricky when we are expected to come in with all the answers and solutions. Just as in any battle, sometimes the best mode of action is to step back, regroup, and come back in with a more strategic approach.

With plant healthcare, some diseases and insects can be treated when called on site throughout the growing season, but with others there is only a brief window when we are able to treat. These circumstances can range from protecting new leaves from being infected by splashing spores, to targeting actively feeding insects during the time the products are fully distributed throughout the tree. Showing up on site after the damage is done can be disheartening for a client to hear, but they can rest assured that if they plan accordingly with you, their tree will be back to its former glory the following year. To help you with some of these time sensitive cases we have listed some of the more commonly encountered pests and diseases with delayed symptoms or treatment strategies.


This is an issue in shade trees where they are unable to properly manufacture chlorophyll. The symptoms can surface as either a pale green color to leaves, yellow leaf margins with dark green veins, or in severe cases, leaf burning. Symptoms may arise as the tree leafs out, or can become more apparent later in the growing season. At RTSA we recommend treating with a combination approach of Verdur, Cambistat, and some soil remediation. Although Verdur can be applied during the growing season, this can cause the leaves to drop and thus stress the tree out more. We recommend holding off on treating until the fall where you can apply the high rate for a longer effect. Teaming this with Cambistat applications and soil remediation can help address a much larger problem that may be going on with the landscape. Soil tests should be done to more accurately diagnose any problems that may be going on in the soil.

Chlorosis Signs
Robert L. Anderson, USDA Forest Service,

Bacterial Leaf Scorch

Trees with this disease can show symptoms beginning around June or July, but usually peaking around August and September. Leaves will develop normally in the spring and as the season progresses, necrosis will move in from the margins towards the petiole. A yellow or reddish band will separate the live and dead tissue. Symptoms can occur in branches or the whole canopy varying in severity from year to year. By the time symptoms arise the treatment window has already passed. For the best results, trees should be treated with Bacastat in the spring after full leaf expansion and prior to July 1st.

Bacterial Leaf Scorch

Apple Scab

This foliar fungus can cause a lot of concern on a client’s property when their crabapple tree becomes almost completely bare by late summer. Apple scab will first show up in late spring as irregular brown/olive colored spots progressing up the canopy and causing the leaves to yellow. Infected leaves will then drop prematurely from late spring through late summer. Applications with Myclotect are best done as the leaves are emerging to protect them from infecting spores being splashed up during rain events. These applications will be repeated throughout the spring infection period every 14-21 days.

Apple Scab

Soft Scale Insects

In general, soft scale insects should be treated for with Transtect in the early spring to target the younger susceptible crawlers. Having the product up in the tree at the time of emergence will insure the best results. However, your clients may notice these pests anytime in the spring or summer and this can impact the results they are expecting from you. What they will most likely pick up on is the sticky excrement called honeydew that will be eventually colonized by sooty mold giving anything underneath the tree an unsightly appearance. Honeydew also has a tendency to attract ants and wasps which can make your client’s backyard a nightmare to live with.

Lecanium Scale signs
Haruta Ovidiu, University of Oradea,

Gypsy Moth

This defoliating caterpillar is best treated right away in the spring with Lepitect just before the leaves emerge. The caterpillars will hatch and immediately begin feeding as the oak leaves unfold sometimes defoliating trees before they ever fully leaf out, and they will continue feeding through early to mid-summer until they pupate. Although you can treat as your clients call in for this one, your best results will come from planning ahead to treat right away next spring. The young larvae are most susceptible to pesticides and become more tolerant of the product as the season progresses. Luckily, most healthy trees can handle a couple years of defoliation before it starts to take a toll on them, so this should help ease your client’s worries.

Gypsy moth caterpillar

Equipped with this knowledge you will now be better able to navigate conversations with your clients as these symptoms arise throughout the season, and in doing so you can begin to sign them up for treatments for the following year. This will help you prepare for work in the off-season now that you have a ball park idea of what to expect. If clients want to protect their trees they can sign up now and be first in line for treatments giving them the best odds for control. They will have peace of mind knowing their trees are in good hands, and in turn you can better manage the hustle and bustle of the spring boom for your plant healthcare business.

For more information on these pests and disease, visit our diagnostics page for detailed diagnostics and treatment guides.

Matt Karst

Matt Karst, Account Development Expert and Technical Support Specialist, Matt is an Account Development Expert and Technical Support Specialist with Rainbow. In his roles, he supports Rainbow's clients all across the country by providing them with education and training. He has a Bachelors of Science in Field Biology from the University of Wisconsin - River Falls, and he has experience as a Field Biologist performing tree/shrub inventories and wetland delineations. His favorite tree is the White Pine Pinus strobus. He enjoys photography, birding, hiking, camping, and gardening. If he were to have a dinner party and could invite any 3 guests, he would invite David Attenborough – to hear about all his stories filming wildlife documentaries, Charles Darwin – to hear his thoughts on evolution and adaptability of nature in an urban world, and Aldo Leopold – to discuss native ecosystems and ecology.