A G N O S T 



UPDATE 



The plant pathogens were certainly busy during 

 luly and August and they haven't shown any signs 

 of reducing their attacks during the early days of 

 Septemberl Although 1 haven't done a tally of all the sannples 

 received in the lab so far this year, it seems that the 

 sample numbers are up from previous years (I'm assuming 

 this is a reflection of an increase in the utilization of the 

 PDL and not an increase in our overall disease problems i 

 The lab was closed for two weeks during August while I at- 

 tended the national meeting of the American Phytopatho- 

 logical Society (APSi and participated in a two-day work- 

 shop on Pythium species identification The meetings were 

 great! I had the opportunity to talk with many of the diag- 

 nosticians from around the country and 'brainstorm' about 

 some of the more unusual diseases we've seen this year 



Several diseases on turf that were problems in lune and 

 early luly continued to be problematic during the rest of 

 the summer Pythium blight (foliar), brown patch and leaf 

 spot (Helminthosporium or Dreschlera) continued to plague 

 the turf industry. Pythium was a particular problem on put- 

 ting greens A interesting note was that several of the 

 greens samples received were showing 'typical' symptoms 

 of Pythium blight, yet Pythium was not isolated. Based on 

 discussions from the APS meetings the general consensus 

 was that several stress factors are playing a role in the 

 symptom development (high leaf temperatures and over- 

 watering, resulting in a lack of oxygen to the roots, were 

 the two most often mentioned). The other interesting note 

 from the meetings was that the majority of the researchers 

 felt that a disease complex was involved: Pythium was not 

 the only fungus present 



In woody ornamentals, the major diseases were bacterial 

 blight on lilacs, anthracnose on maples, brown rot shoot 

 blight on Prunus species, and leaf spots on several different 

 hosts Two cases of Phytophthora dieback were confirmed 

 on rhododendrons and one case of dogwood anthracnose 

 was diagnosed The high temperatures and low rainfall 

 caused heat stress problems such as scorch in several loca- 

 tions The early heat and low rainfall also resulted in sig- 

 nificant losses in newly planted Christmas trees 



One of the major problems on annuals and perennials 

 was damage caused by four-lined plant bugs and lace bugs 

 It seems these two 'critters' really thrived during the hot 

 spells Fusarium and Verticillium wilt were diagnosed on 

 several crops (sweet pea. asters, and larkspur) The rapid 

 development of symptoms and wilting was once again en- 

 hanced by the high temperatures and water stress As ex- 

 pected. Botrytis was a problem throughout the summer 

 thanks to the high relative humidity The flowers of dahlias 

 and marigolds were particularly hard hit by Botrytis blight 

 Powdery mildew began showing up in mid-luly (or earlier) 

 on flox, lilac, and azalea, to name a few 



The most important thing to consider during the next 

 two months is preventative measures to reduce the impact 

 of diseases next season SANITATION is the most impor- 

 tant Remove all crop debris and destroy it if you know 

 that diseases were a problem Composting often does not 

 kill the pathogen in the debris: thus when it is used as a 

 mulch, the pathogen is reintroduced into the crop Be sure 

 to protect woody ornamentals against winter damage and 



desiccation. Now is the time to apply fungicides for control 

 of Rhizosphaera needlecast. The time to apply fungicides 

 for the control of snow molds on turf grasses is early-mid 

 November. Last year, downy mildew occurred on snap- 

 dragon during early October. If you are growing snaps, you 

 want to be sure to scout the crop and get a fungicide on at 

 the first sign of the fungus on the undersides of the leaves 

 The lower leaves of poinsettias should also be examined 

 regularly for powdery mildew Look for small chlorotic or 

 yellow lesions on the upper surface of the lower leaves, 

 then check the underside for the white-to-gray growth of 

 the fungus. It is good practice to carry a small zip-loc bag 

 around to put infected or suspicious leaves in If any pow- 

 dery mildew infeaions are found, fungicides should be ap- 

 plied immediately — be sure to get good coverage on the 

 undersides of the leaves A final reminder is to monitor 

 your geranium crops for bacterial blight A random selection 

 of plants can be placed in an area where the night-time 

 temperatures can be maintained at 27 C (80 F). It takes at 

 least four weeks for symptoms (wilting, v-shaped yellow ar- 

 eas of the leaves) to appear 



If you wisk tc submit plant material to the PDL for diagnosis, send 

 samples to Dr. Cheryl Smith. Plant Diagnostic Lab. Nesmith Hall, 

 UNH, Durham. NH 03824 Samples should be accompanied by an 

 identification form {available from your county Cooperative Extension 

 office). There is no fee [at this time] Cheryl Smith is the UNH Co- 

 operative Extension Specialist in Plant Health, and can be reached at 

 (603) 862-3841. 



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26 



The Planisman 



