SURVIVAL MECHANISMS OF PHYTOPATHOGENIC BACTERIA .

JRF PLANT SCIENCE EDUCATION TECK

BSC AGRICULTURE.NOTES

INTRODUCTION

Natural habitats usually do not provide bacteria the continuity of agricultural crops. With continuous culture. perpetuation of pathogen is no problem. Although agricultural practices provide some discontinuity between crops. it is less than that in nature. Uniformity of crop germ plasm also favors inoculum buildup and perhaps perpetuation of the pathogens. The growth of most plant pathogens is discontinuous, because of the seasonal effect upon either the pathogen or the host. A successful pathogen must be able to bridge discontinuities, such as the gaps between successive crops and seasons. To reestablish when conditions are again favorable, inoculum must survive. Facultative saprophytes or facultative parasites are not as handicapped by discontinuous growth as are obligate parasites. Discontinuous growth of the pathogen mainly decreases the amount of inoculum (bacteria that cause disease when placed in suitable contact with the host). The success of a bacterial plant pathogen depends in part on the amount of inoculum (bacterial cells) it produces. Because bacteria have a short generation time, a small amount of surviving primary inoculum can rapidly produce an epidemic. What is the minimum amount of inoculum necessary to initiate disease? Enough for mere survival is not necessarily the answer since transmission to a host is necessary. The source, exit, and transmission of primary inoculum are all necessary for occurrence of disease, for survival, and for continuity of the bacterial species. Establishment of a "curtain" between host and pathogen may be simpler and cheaper during the primary inoculum phase than thereafter: The longevity of primary inoculum is important in the success of bacterial pathogens and depends upon its ability to escape or endure adverse environmental conditions (62). Survival varies with the form of primary inoculum and may depend on external factors as well as on the internal makeup of the pathogen. Because there are about 200 different species of bacterial plant pathogens. variations in modes of 199 Annu. Rev. Phytopathol. 1974.12:199-221. Downloaded from www.annualreviews.org Access provided by 2402:8100:238d:4916:d438:10ef:1d5b:e945 on 06/09/20. For personal use only. 200 SCHUSTER & COYNE survival are certain to occur. The species included in this paper are limited to those that illustrate concepts and principles. Actimomycetes are excluded (14). There are five genera of bacterial phytopathogens: Agrobacterium, (A.), Corynebacterium (C), Erwinia (E.), Pseudomonas (P.), and Xanthomonas (X). They are aerobic non-spore-forming rods. Only Corynebacterium is Gram-positive. Most are motile with polar or peritrichous flagella, but a few are atrichous. Phytopathogenic bacteria do not form resting spores or structures comparable to fungi or nematodes; they remain dormant during the quiescent period in association with the following animate or inanimate agencies: (a) seeds, (b) perennial plant hosts or parts, (c) insects, (d) epiphytes, (e) plant residues, (J) soil and other nonhost materials. Longevity of pathogens in these agents under natural and artificial environmental conditions are discussed in this review, and selected references are provided.

ASSOCIATION WITH SEED

"The flowers of all the tomorrows are in the seeds of today!" (22). The term seed is here used in a popular sense, and includes fruits, such as caryopses and achenes, but excluces vegetative propagules (e.g. seed potatoes). Seeds are an ideal agency for survival of plant pathogens when the growing host is lacking. Orton (74) suggested in 193 1 that any bacterial phytopathogen is likely to be transmitted by seed; he listed 59 such pathogens of the 1 28 species then described. In an annotated list of seed-borne pathogens, Noble & Richardson (73) included 95 species and varieties of bacteria, about half of the phytopathogenic species described. Baker & Smith (6) and Baker (5) outlined the essentials of the subject, referring to only a few bacterial species.

Sl.No	Disease	Bacteria
	a. Seed
1.	Bacterial canker of tomato	Clavibacter michiganensis subsp. michiganensis
2.	Goss’s bacterial wilt and blight of maize	Clavibacter michiganensis subsp. nebraskensis
3.	Bacterial wilt of bean	Curtobacterium flaccumfaciens pv. flaccumfaciens
4.	Bacterial brown stripe of rice	Pseudomonas avenae
5.	Bacterial grain rot of rice	Pseudomonas glumae
6.	Bacterial blight of soybean	Pseudomonas syringae pv. glycinea
7.	Angular leaf spot of cucurbits	Pseudomonas syringae pv. lachrymans
8.	Halo blight of bean	Pseudomonas syringae pv. phaseolicola
9.	Bacterial blight of pea	Pseudomonas syringae pv. pisi
10.	Black rot of crucifers	Xanthomonas campestris pv. campestris
11.	Bacterial blight of cotton	Xanthomonas axonopodis pv. malvacearum
12.	Common blight of bean	Xanthomonas campestris pv. phaseoli
	b. Planting material
1.	Ring rot of potato	Clavibacter michiganensis subsp. Sepedonicus
2.	Silvering of tulip	Curtobacterium flaccumfaciens pv. portii
3.	Bacterial wilt of carnation	Pseudomonas caryophylli
4.	Gladiolus scab	Pseudomonas gladioli pv. gladioli
5.	Bacterial wilt of potato and ginger	Burkholderia solanacearum
6.	Bacterial leaf spot of Photinia glabra	Pseudomonas syringae pv. photiniae
7.	Leaf scald and white streak of sugarcane	Xanthomonas albilineans
8.	Bacterial leaf spot of Begonia	Xanthomonas campestris pv. begoniae
9.	Yellows of hyacinth	Xanthomonas campestris pv. hyacinthi
10.	Bacterial blight of iris	Xanthomonas campestris pv. tardicrescens