AGNOSTIC UPDATE 



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It certainly looks like the saying 

 "when it rains, it pours" held true 

 for the month of October (particu- 

 larly for those of us in the Seacoast 

 area)! The greatest threat to plant 

 health due to flooding will probably 

 be root asphyxiation. Luckily, the 

 flooding occurred at a time of year 

 when the temperatures are cooler 

 (root asphyxiation occurs more rap- 

 idly with higher temperatures) and 

 most of the plants are entering dor- 

 mancy. Several conifer species (in- 

 cluding eastern white pine, hemlock 

 and spruce) are fairly intolerant of 

 waterlogged soil conditions for pro- 

 longed periods. We can expect to 

 see some decline or possible mor- 

 tality if these species remain in 

 flooded soils for several weeks. 



During the last two months, there 

 have been only a few problems 

 worthy of note from samples sub- 

 mitted to the UNH-PDL. BoTRYTIS 

 CANKER on poinsettia has been 

 causing problems for a few growers 

 and PHYTOPHTHORA root and crown 

 rot was diagnosed on Lamium. Dis- 

 eases and symptoms associated 

 with drought stress continued to ap- 

 pear on woody plant hosts. Rela- 

 tively speaking, however, things 

 have been fairly quiet. 



There are several common prob- 

 lems we should watch for during the 

 next few months. Pythium ROOT 

 ROT is a common problem occurring 

 on a wide range of plant material, 

 including geraniums and bedding 

 plants. The initial symptoms may in- 

 clude yellowing or symptoms usu- 

 ally associated with nutrient defi- 

 ciencies: stunting, marginal browning 

 of the leaves, and — eventually — 

 wilting. Remove symptomatic plants 

 from the containers and check the 

 roots. Roots infected with Pythium 

 appear brown and/or water-soaked. 

 The cortex of the root is easily 

 pulled off, leaving the stringy stele. 



Severely infected plants should be 

 discarded. The remaining plants 

 should be treated with a fungicide 

 drench. The potting mix should not 

 be reused, and the containers 

 should be disinfected before reus- 

 ing. Wet conditions favor the devel- 

 opment and spread of pythium, so 

 avoid over-watering during extended 

 cloudy periods and avoid splashing 

 the mix from plant to plant when 

 watering. Preventative fungicide 

 drenches such as Banrot are often 

 recommended for geraniums and 

 chrysanthemums. Care should also be 

 taken to keep the ends of hoses off 

 the greenhouse floors. 



BoTRYTiS BLIGHT is another dis- 

 ease that is prevalent on green- 

 house crops during the winter 

 months. High relative humidity fa- 

 vors the disease, thus techniques 

 aimed at lowering humidity levels 

 (e.g., venting) and improving air cir- 

 culation (e.g., plant spacing) are 

 usually effective in reducing the 

 damage caused by botrytis. Strict 

 sanitation, particularly removing se- 

 nescent plant tissues, can also help 

 to reduce botrytis blight. 



Impatiens necrotic spot virus 

 (INSV) is always a threat to green- 

 house crops, especially impatiens. 

 New Guinea impatiens, gloxinia, and 

 cyclamen. Vegetables transplants, 

 particularly tomatoes and peppers, 

 are also hosts for the virus. INSV is 

 thrips-transmitted, most commonly 

 by western flower thrips (WFT). In- 

 spect new stock carefully for thrips 

 and use blue or yellow sticky cards 

 to monitor for thrips (more than 5- 

 10 WFT per card per week seems to 

 be a useful threshold). If insecti- 

 cides are necessary for thrips con- 

 trol, be sure to rotate between 

 chemical classes every 3-4 weeks to 

 reduce the chance of resistance 

 building up in the thrips popula- 

 tion. Plants infected with INSV can- 



not be cured, and should be de- 

 stroyed. Suspicious plants should 

 be isolated from the rest of the 

 crop and sample plants should be 

 submitted to the UNH-PDL (or an- 

 other lab) for testing, since several 

 other controllable diseases can 

 mimic INSV symptoms. 



Geraniums should be monitored 

 for BACTERIAL BLIGHT. The most 

 common symptom is wilting of one 

 or more leaves even though the soil 

 is moist. Leaf spots may develop 

 when the bacterium is splashed 

 from one plant to another or drips 

 from hanging baskets onto plants 

 below. Yellowing, often in a V- 

 shaped pattern, is also a common 

 symptom. The symptoms on ivy ge- 

 raniums are not as obvious as those 

 on seed and zonal types. On ivy ge- 

 raniums, the symptoms are easily 

 confused with edema. If bacterial 

 blight is suspected, a laboratory di- 

 agnosis is necessary to confirm the 

 presence of the bacterium. If con- 

 firmed, (I) plants cannot be cured 

 by fungicide sprays or drenches, (2) 

 infected plants should be destroyed 

 and soil should not be reused, (3) 

 suspicious plants should be iso- 

 lated, and (4) tools, pots, flats, and 

 bench tops should be sterilized 

 with 10% bleach solution. 



I hope ail of you have a happy, 

 prosperous, and plant-healthy holi- 

 day season! 



If you wish to submit plant material to 

 the UNH-PDL for diagnosis, send samples 

 {with a check for $12.00) to-. The UNH 

 Plant Diagnostic Lab. C/O Dr. Cheryl 

 Smith, Plant Biology Department, 241 

 Spaulding hall, UNH, Durham, NH 

 03824. Samples should be accompanied 

 by an identification form {available from 

 your county Cooperative Extension office). 

 Cheryl Smith is the UNH Cooperative 

 Extension Specialist in Plant Health and 

 can be reached at (603) 862-3841. 



THE PLANTSMAN 



