DIAGNOSTIC UPDATE 



Well, if it's not one thing, it's an 

 other! Lack of water seems to 

 be the problem this year (at least 

 until the rains in early May). But 

 we haven't hit June yet. ..let's hope 

 it isn't a repeat of last year. The 

 cooler temperatures kept things 

 rather slow during April, but the 

 buds and flowers really popped once 

 the weather warmed over the first 

 two weeks of May. If we stay rela- 

 tively dry, the disease problems of 

 last year will hopefully become just 

 a memory. 



Woody Ornamentals 



Compared to last year, this has been 

 a relatively dry season during bud 

 break and leaf expansion. Thus, the 

 incidence of anthracnose and blight 

 should be much less than last year 

 (although we can always expect 

 some anthracnose on the typical 

 hosts such as maple and sycamore). 

 Although I haven't yet seen any 

 samples of Ascochyta leaf blight on 

 lilacs, it's worth repeating the de- 

 scription of the disease since it was 

 the most common problem on lilacs 

 last year. Ascochyta has two phases, 

 a shoot blight in the spring (which 

 resembles bacterial shoot blight) and 

 a foliar phase causing necrotic leaf 

 spots and marginal blight during the 

 summer and early fall. Dead shoots 

 and branches should have been 

 pruned prior to bud-break and fun- 

 gicides should be applied through 

 leaf expansion (copper fungicides 

 combined with a thiophannate- 

 methyl product work well). Thin- 

 ning dense bushes will also help re- 

 duce disease severity. 



Shoot blight of Prunus species, 

 caused by the fungus Monilinia, has 

 been very damaging the last several 

 years, so protective fungicide sprays 

 may be warranted on nursery stock 

 and valuable landscape specimens. 

 Fungicides should have been first 

 applied when the blossoms opened 

 and followed by a two additional 

 applications ten days apart. If blight 



does occur, dead shoots should be 

 removed during dry weather to pre- 

 vent further spread of the disease. 



Cedar rust galls were producing 

 telial horns (the jelly-like, orange 

 growths) during the rainy period of 

 May 4-8. Susceptible deciduous host 

 (apple, quince, Amelanchier, haw- 

 thorn) could have been infected dur- 

 ing this time. In most cases, control 

 consists of eliminating the alternate 

 host or applying fungicides to the 

 deciduous host (practical for nurser- 

 ies and orchards only, due to cost 

 effectiveness) . 



Greenhouse Crops 



Since bacterial blight and South- 

 ern bacterial wilt of geraniums seems 

 to be on everybody's mind this 

 spring, I thought it would be worth- 

 while to point out the similarities 

 and differences between bacterial 

 blight, caused by Xanthamonas 

 campestris pv. pelargonii and South- 

 ern bacterial wilt, caused by 

 Pseudomonas solanacearum. 



Similarities: 



1. Both bacteria can cause gerani- 

 ums to become chlorotic and/or de- 

 velop necrotic (dead) wedges in the 

 leaves, develop totally necrotic 

 leaves, or wilt. 



2. Both bacteria can kill geraniums. 



3. Both need to have infected/symp- 

 tomatic plants rogued out as soon as 

 possible after confirmation by a di- 

 agnostic lab (testing for Pseudomonas 

 takes a couple of days longer than 

 testing for Xanthamonas). 



Differences: 



1. Only Xanthamonas (bacterial 

 blight) can cause l/l6"-l/8" diam- 

 eter, round, brown spots. 



2. Xanthamonas (bacterial blight) 

 only infects plants in the 

 Geraniaceae (geranium) family, while 

 Pseudomonas (Southern bacterial 

 wilt) has a wide host range, espe- 

 cially foliage plants. 



The most important thing to do j 

 when suspicious plants are first no- i 

 ticed is to send plants for testing 

 and isolate the remaining symptom- ] 

 atic plants. I have tested (via Agdia | 

 Testing Labs) over 80 samples this 

 year for bacterial blight and all were I 

 negative. The dozen samples tested ' 

 for Pseudomonas were also negative. ' 

 It's important to remember that in- 

 fected plants can't be cured. \ 



Verbenas have had several prob- | 

 lems this spring. The most common j 

 problem has been Phytophthora I 

 stem canker and root rot. The j 

 symptoms include brown cankers at ] 

 the base of the stems, wilting, and 1 

 root rot symptoms similar to 

 Pythium (the cortex of the root 

 sloughs off easily when pulled gently 1 

 between the thumb and forefinger). I 

 Infected plants should be removed 

 and the remaining plants should be I 

 drenched with fungicides used for 

 control of Pythium. Ascochyta leaf 

 spot and blight has also been com- 

 mon. The symptoms include ne- ; 

 erotic leaf spots and stem browning | 

 similar to that caused by Phyto- | 

 phthora. Avoid wetting the foliage 

 when watering and space plants to 

 allow for good air circulation. 



Alfalfa mosaic and cucumber mo- 

 saic virus were both diagnosed on 

 Nemesia. The symptoms included 

 distorted foliage, white line patterns, 



and ringspots. Virus-infected plants j 



should be destroyed (don't take cut- \ 



tings from infected plants). Other dis- ] 



eases and problems that have been di- i 



agnosed this spring include Pythium I 



root rot on osteospermum, iron and | 



manganese toxicity of geraniums in- | 



duced by low pH, crown gall on \ 

 argeranthemum causing stem galls, 



and lots of edema on ivy geraniums. ,; 



Herbaceous Ornamentals I 



Botrytis blight was the most signifi- j 



cant problem during June and early ' 



July of 1998, particularly on bed- j 



ding plants, Asiatic lilies, and peo- j 



nies. Be sure to adequately space to 1 



