We’ve had a very rainy summer here in Michigan this year. Raining at least once a week, and sometimes 4-times a week. As the climate shifts, so too do the things I see on trees. An interesting metric for climate change.

A number of weird and uncommon things are popping up as a result this rainy summer, lots of anthracnose, some oak-leaf blister, but most surprisingly, I’m seeing lots of scorch on leaves.

We’ve had a few 90 degree days here and there, but this hasn’t been a particularly hot summer. Generally we expect heat without rain to be the cause of widespread scorch. And with a word like ‘scorch’, it is easy for us to think ‘hot’ or ‘dry’.

Some acute cases of scorch are easy to explain: a bonfire that gets a bit too large may scorch a tree, for instance. A constantly blowing AC unit may also scorch a nearby tree. These are intuitive and easy to solve puzzles.

I’m finding leaf scorch symptoms across a variety of different tree species in different locations. It is especially surprising consider it has not been dry. It’s been extraordinarily wet.

So why am I seeing scorch everywhere?

Initially I did not even think about scorch when seeing these symptoms. With how wet things were, I assumed something moisture related like fungi or bacterial, right?

Scorch is sort of like a gateway symptom. Sometimes it can indicate something very simple with a simple fix. Other times it might indicate the beginning of something severe. 

Scorch is generally necrosis along the margins of the leaf, moving inwards towards the main vein of the leaf. Contrast those symptoms with the different ones below:

The above symptoms are meaningful things to trees, but are not in the same symptom category as scorches are. Generally, scorches are caused by abiotic factors, like water or lack thereof (with the exception of bacterial leaf scorch). Each of the above symptoms are biotic in origin, caused by pathogens. Those are not scorches. Each of those examples has a different pattern or cluster of patterns that gives its cause away.

Two seemingly opposite problems causing the same symptom

Tree leaves can scorch from both too little water, and too much water. Isn’t that odd? At a glance it is to me too. But take a peak at the equations below:

Really simplifying here. Sunlight, carbon dioxide, and water are the ingredients for photosynthesis, which creates glucose and oxygen as byproducts. Respiration, on the other hand, takes that glucose and oxygen produced by photosynthesis and creates carbon dioxide, water, and ATP (a unit of chemical energy). These two processes are intertwined, and are nearly exactly the reverse of one another. Both need to function optimally for plants to function properly.

In one case, dry scorch is caused by the deficiency of water, disrupting photosynthesis, pictured below:

Without adequate water, photosynthesis cannot occur. And the scorch symptom appears in areas where water depletes the fastest, at the margins of the leaves.

Another case, soggy scorch (I just made up that term lol), is caused when there is deficiency of oxygen in the soil. This disrupts respiration of the roots that supply the water to the leaves for photosynthesis.

Soggy scorch, caused by excessive rain, displaces all of the oxygen from soil pore spaces and replaces it with water. Roots cannot respirate in the absence of oxygen. Trees actively and passively absorb water. The active uptaking of water costs ATP. In the absence of available oxygen, ATP cannot be readily replaced, and the tree cannot uptake water. And the scorch symptom appears in areas where water depletes the fastest, at the margins of the leaves.

In either instance of dry scorch or soggy scorch, the source is ultimately the same: a lack of water to fund photosynthesis. You might call scorch a symptom of ‘hydraulic dysfunction’, and when working with trees, it is meaningful to try to figure out its source.

Can you tell soggy scorch from dry scorch?

By the symptoms alone, it might be difficult. But with context, you can figure it out.

If the problem is dry scorch, watering is a pretty simple solution. Chronic droughts certainly cause tree mortality. Mild droughts can still be stressful and can predispose trees to other meaningful disruptions (xylem cavitation, to name one). Remediating this is fairly easy and intuitive: water the soil. Adding humectants to soils where water restrictions are in place can help soils retain moisture for longer. This is especially important in more arid climates.

Sometimes the soil is to blame for soggy scorch. Soils very high in clay content tend to stay wetter for longer (and can become hydrophobic if they totally dry out). Decompaction, introduction of organic material, and aeration are feasible long-term efforts to be made against soils prone to sogging up. But there aren’t really short term, immediate responses to soggy scorch like there are for dry scorch.

It is challenging to work against soils, but it is even harder to work against the climate. If trees are showing symptoms of soggy scorch here or there, once every few rainy years, no big deal. Shit happens, and trees are tough. Temporary sogginess is no biggie. But if periods of excessive rain are causing some scorching symptoms chronically year after year, this might be meaningful.

Conditions that create soggy scorch in leaves can also create environments favorable for pathogenic oomycetes like pythium or phytophthora species (root rotting organisms) in the soil. Once these organisms are established, addressing them can be critically important to the tree’s survival. If conditions persist, if the environment or soil environment stays favorable for pathogens, long term management plans ought to be considered.

Phosphoric acid trunk sprays can be used to ward off some symptoms, and basal drenches of Mefenoxam can suppress these organisms too.

SO YEAH, it is meaningful to know what you’re looking at. Dry scorch or soggy scorch. And it is interesting to be try to interpret what the heck trees are doing.

Other things can cause scorch too

Anything that can cause a similar disruption to either the tree’s photosynthesis or respiration can, in theory, cause scorch symptoms.

Extreme nutritional deficiencies are one such issue.

The maple pictured here is showing extreme iron or manganese deficiency. These symptoms could be caused by the absence of this micronutrients, or by a pH issue. In either event, this is also abiotic in nature like the other scorches.

In my region, this symptom is common. In extreme cases, the nutritional deficiency symptoms of the yellowing of the interveinal space coupled with scorch, suggests that this is likely due to the deficiency. It is possible that water-related scorch affects the tree too.

Physical vascular disruption can also caused scorch symptoms to appear. This can be caused by self girdling or severe root damage.

The case above shows some acyanic leaves during the summer time. The rest of the tree’s stems held leaves that looked normal, but the girdled stem held leaves that were acyanic. This reddening on its own does not always precede scorch, but in extreme cases of vascular disruption, it can be an early sign of scorch or even leaf mortality.

Check out the linked article about acyanic leaves, that function is fascinating in its own right.

And lastly, there is a biotic source of scorching: bacterial leaf scorch. I don’t have any photos of this, I have yet to run into it. Although I have read that managing symptoms is possible, but no effective or pragmatic approaches for control have yet to be reached.

Typical summer science article from me, yeah? Usually more concise than some of my other articles. I like writing these, though they appeal to an even narrower audience than the rest of my works.

So if you’re reading this far, thanks. I appreciate ya.