I am sorry for the delay in responding. Our website was down for awhile today for service updates.
With 50 trees at stake, I can certainly understand your concern.
There can be several things going on with your trees. Below are some of the possibilities although I suspect that due to the timing of the discolored needles (late summer into fall) you are either seeing the effects of the drought, seasonal needle drop or both. More info:

• Drought: This last summer’s 2021 drought was particularly hard on trees and shrubs. If you look at the map for August 24, 2021 Brainerd was under a severe drought. 2020 wasn’t much better so many trees and shrubs went into 2021 already at a loss. During dry weather it is very important to water your trees and shrubs and to do so right up until the ground freezes. I am not sure how much water your trees were receiving. A general guideline is that if they are not receiving an inch a week from rain, they will need supplemental watering. Here are some sites with more information on watering:
  • Watering trees in dry weather: 
    • Watering established trees and shrubs
      
• Seasonal Needle Drop on Evergreens:
  It is common for older inner needles of evergreens to drop after one or more years. Sometimes this happens all at once, or they could drop slowly, usually in late summer or early fall which is why I am suspicious that this could be affected your trees. Note the part of that states, "In times of drought, needle browning may be particularly noticeable, because more needles are shed in response to environmental stress. White pines show the most dramatic needle drop change."  The photos in the gallary of the site for seasonal needle drop from the Morton Arboratem look very similar to yours. 

This process of seasonal evergreen drop is natural. You can brush out, or the wind will blow out, the yellow/brown needles and newer replacement growth will push in late this spring to early summer.

Here are some deciding factors to help understand if the needles are turning yellow/brown from insect damage or from disease: the needles should be uniformly yellow and free from fungal spots. The newer grow at the tip of the branches is usually green-usually it is the inner portion of the tree that is affected during needle drop. Examine the needles carefully to help your diagnosis. Occasional spots and blemishes are natural but extensive spots requires a closer look. See the section on disease or insects below.

• Disease or insects: the link below will include photos of the most common diseases/insects effecting pine trees. Pay close attention to the section on needles.

Follow the above guidelines for watering and give those trees plenty of water this spring.  Since you have quite an investment to protect if your trees do not show that they are rebounding later this spring/early summer it might be wise to hire a professional arborist or forester to determine their health and further management.

Hiring a tree care professional - Finding and hiring a professional arborist or forester can help you diagnose problems and get professional care for your landscape trees.

I hope that 2022 brings us more moisture and that your trees return to normal.
Kathy

Bill,
This page with more info on white pine blister rust has some tips that might help you identify the disease.  Check the branches with the dead needles looking for cankers, swollen area with cracked bark (see photos).  You mentioned the white sticky sap, but didn't say anything about spores. This spring watch for white and yellow blisters at the edge of the canker that will release orange spores. 
When watering your trees, be sure that you water at the base and that you are not watering the needles as disease is carried by moist air currents.
In addition to arborists, the DNR division of forestry might be a good resource to assist you with the scale of disease in Crow Wing County and the amount of trees that they are seeing affected by the drought.  I found two offices in your area:
DNR, Division of Forestry in Aikin:<personal data hidden> and 
DNR, Division of Forestry in Little Falls:<personal data hidden>
Let me know how this works out.

Hi Bill,
I forgot to ask you if you have any Ribes species (gooseberries,currants) nearby your white pines?  Check the underside of their leaves when able to look for evidence of infected leaves.  More info on this is available on the site I previously gave you.  There are resistant cultivars that you can plant.  
I also checked with my sister who worked for the US Forest Service out of Ely.  She said that the DNR Forestry department should be able to assist you further or give you a contact of someone in your area to help.  

I am not sure of that, and I will check as soon as spring arrives. I don’t think I’ve planted any, and I did ask a neighbor as well. How long does it take for a newly planted gooseberry to start infecting trees?

Sent from my iPhone



White pine blister rust needs both the white pine and a ribes plant (red, white or black currant or gooseberry) to complete it's life cycle.  If you live in a rural area the ribes plants may nearby as the spores are carried by the wind.  You asked about the stages of infection which are detailed within this article, esp. this section:

7.  "In the first summer after infection of the pine tree, gummy, orange droplets full of fungal spores may be seen on branch cankers.

8.  The second spring after infection, white, blister-like structures form at the edge of the canker. These fungal structures (aecia) crack open to release powdery yellow orange spores called aeciospores.
9. These spores can be carried long distances on wind currents to infect Ribes spp."