Photo of the Week

Dwarf mistletoe on spruce

Witches' brooms on black spruce caused by Arceuthobium pusillum.

Gall on aspen

Cross section of an aspen with gall on the main stem. These abnormal growths are likely initiated by Agarbacterium tumefaciens. The gall consists of hyperplasia and hypertrophy of xylem cells which produce unusal growth patterns in the wood.

Davy with rhizomorphs

Subcortical rhizomorphs produced under the bark of tree killed by Armillaria and part of Davy's collection for class.


Heterobasidion irregulare fruiting bodies at base of tree. This pathogen has recently been found in southeastern Minnesota and is expected to spread as it has done in Wisconsin. WI DNR reports 28 counties in Wisconsin with Heterobasidion Root Disease. Be on the look out for this pathogen in Minnesota.

zone lines in wood

Cross section of a decayed tree showing zone lines (pseudosclerotial plates) also called spalted wood formed by different incompatible populations of fungi.

decayed tree

Cross section of a tree with decay. Tree defences compartmentalize decay allowing the tree to continue to grow. However, this tree was hazardous when standing. 

Chicken of the woods

The Sulfur Shelf or Chicken of the Woods fruiting on an oak. This fungus causes a brown rot and hazardous conditions in the tree.

brown rotted wood

Brown rotted wood has a loss of wood strength early in the decay process. Here is some advanced decay that easily crushed with just slight pressure.

Pine oak gall rust

Aecia of pine-oak gall rust or also called eastern gall rust. The entire fusiform swelling can produce masses of aeciospores in the spring. These spores infect red oak leaves.  

White pine blister rust aecia

Aecia of white pine blister rust is found on infected white pines in late may to early June in Minnesota. These spores infect currents and gooseberries.

Lophodermium ascospores

The ascospores of Lophodermium as seen under the microscope. The spores are long and tread-like with a sticky sheath that helps adhere them to needles after they are ejected out of the hysterothecium.

DED Campus Before

The University of Minnesota St Paul Campus before Dutch elm disease started to take its toll. Big elms lines all the campus streets. This picture shows where the buses stop outside of the student center.

canker on ash

What fungus do you think caused this canker on ash?

maple anthracnose

Is my tree Dying? This homeowner was very upset to see his maple looking so bad and thought it was going to die. This is maple anthracnose and in wet, cool spring environments it can cause a lot of infection and defoliation. Although trees look bad and can be stressed due to the defoliation, most trees recover.

Penicillium spores Bread mold

Chains of conidia (asexual spores) produced on conidiophores from a mold (Penicillium) that was growing on some old bread I forgot to eat.

Iron Chlorosis on oak

There are many different types of abiotic or non-infectious agents causing tree problems. This pin oak is located in downtown St Paul. What is causing this tree to decline?

red pine trees

Keeping forest and urban landscape trees healthy requires a good knowledge of tree diseases. This class will provide you with important information you can use in the future. Watch this page for new photos each week.