MARKET DISEASES OF FRUITS AND VEGETABLES 23 



a greasy yellowish slime. This finally dries, forming a yellowish- 

 brown crust on the surface. In severe cases the bacteria penetrate 

 the pod and attack the seeds. Lesions on the seed usually are small, 

 irregular, yellow spots or blotches; in severe infection the whole seed 

 may become shriveled and yellow. 



In the field, bacterial blight on the leaves is first indicated by 

 small, irregular, water-soaked areas, bordered by reddish lines. Later, 

 large areas of the leaves may become involved, finally drying out 

 and becoming brown and brittle with age. Bacteria washed down 

 from the cotyledons or leaves sometimes produce reddish-brown 

 lesions on the stem of the plant, or produce water-soaked areas 

 that enlarge and girdle the stem at the nodes. Plants thus affected 

 often break over at the diseased nodes. In continued muggy weather 

 badly affected plants may have the appearance of being scalded; the 

 pods do not fill out and the ripening is uneven. 



CAUSAL FACTORS 



The pathogen of the common form of bacterial blight, Phytomonas 

 phaseoli, is carried over from year to year in or on diseased or con- 

 taminated seed, or in bean trash that remains in the soil. The grow- 

 ing seedlings bring the bacteria above ground in the developing 

 lesions. During damp warm weather the bacterial masses ooze out 

 of these tissues, thus making possible the spread of the bacteria by 

 insects, water, and other contact agencies. The bacteria are very 

 resistant to sunlight and drying. Consequently they are readily 

 disseminated by the wind in fragments of leaf lesions and with soil 

 particles and can lie dormant until proper moisture conditions favor 

 their growth and entrance into plants through the stomata. 



Pods that develop the disease in transit do not necessarily show 

 symptoms at time of harvesting or loading. Incipient lesions may 

 develop in transit, but there is no evidence of any new infection 

 after harvest. 



A form of bacterial blight usually spoken of as halo blight, caused 

 by Phytomonas medieaginis var. phmeolicola, is becoming increas- 

 ingly important in many bean-growing regions. This disease often 

 is more serious and causes more wilting of plants than the com- 

 mon form. Many symptoms are practically identical in the two 

 diseases, but usually the halo blight can be distinguished by the 

 grayish-white bacterial exudate on the leaf and pod lesions and 

 the characteristic light or pale-green halo surrounding the lesions. 

 On the market the pod spots generally show more water soaking, 

 are usually more circular in outline, and they remain greenish gray 

 instead of taking on the yellow and reddish-brown shades typical of 

 the common bacterial spot (pi. 7, G). 



CONTROL MEASURES 



Crop rotation and the planting of disease-free seed are the usual 

 recommendations for the control of bacterial blights. However, 

 by selection and breeding some fairly resistant varieties have been 

 obtained. Although no varieties show absolute immunity, some of the 

 Refugee types exhibit a high degree of resistance. Unfortunately, 



