News from Elsewhere

Nursery inspection update

by Steven Shimek
Minnesota Department of Agriculture

The Minnesota Department of Agriculture Nursery Inspection and Certification program inspects over 8000 acres of nursery stock grown for sale and certifies it to be free of injurious plant pests.  These annual inspections facilitate movement of pest free stock within Minnesota and allows stock to be shipped interstate and internationally.  This program also monitors nursery stock shipments from outside Minnesota to assure that stock entering the state is properly certified for freedom of pests by the shipping state.  Inspection staff reviews shipping documents and conducts inspections to verify compliance with state and federal quarantines which include:  gypsy moth, pine shoot beetle and Japanese beetle.  

 Nursery stock dealer inspections

This spring no quarantine plant pests were intercepted.  Disease problems including Rhizosphaera needlecast on Colorado blue spruce, Phomopsis Juniper blight on Juniper,  Cedar-quince rust on Cockspur hawthorn and various foliar fungal diseases were commonly found in dealer sites.  Non-viable or dead stock was identified and required to be removed from sale.  Rose mosaic virus was found on shrub roses.

 Nursery stock grower inspections

Grower inspections began in June and continue into October.  Stock that is infected or infested with an injurious plant pest does not meet certification requirements is placed under quarantine pending treatment, control and reinspection.  Rhizosphaera needlecast and Cytospora canker has been found often on Colorado blue spruce.  Phomopsis and cedar-quince rust were also found in growing sites.  Trees found with linden borer, bronze birch borer, Nectria canker and various other cankers were ordered destroyed.  Scots and Austrian pine infested with Zimmerman pine moth were placed under quarantine and ordered to be burned or buried.

Mystery condition

For the past few years, a mysterious condition affecting Black Hills Spruce, Picea glauca densata, has been reported from several locations around Minnesota.  Distorted growth with needles that seem to have been cemented together have made many nursery trees unmarketable.  These symptoms provided no signs of insect or disease involvement.  Despite consultation with many insect and disease diagnosticians, the cause of this distortion has remained a mystery.   In late summer 1998, samples taken from symptomatic trees were examined under 40X magnification revealing very tiny mites, referred to as eriophyids (aero-PHY-id).  These mites are very widespread, but relatively little is known about them.  They are very hard to see, even with a 10X hand lens, making field identification difficult.   In Minnesota they have been found on  white spruce and Colorado spruce.   Elsewhere, they have been reported on Douglas-fir, most species of fir, hemlock, juniper, pine and spruce.

To confirm the diagnosis that eriophyid mites were causing the distortion, mites needed to be recovered consistently from symptomatic spruce trees from different locations.  In 1999, many samples were collected but yielded no visible mites or other evidence of their presence.  Hope of solving this mystery turned to frustration.  After a few weeks, the samples began to deteriorate, some growing mold.  However, one last look before the trash can revealed hundreds of eriophyid mites had hatched on one small sample.  Inspection of subsequent samples revealed mite populations may be elusive to see, but given some incubation time, are present in significant numbers to cause damage we have been seeing in the field.