MARKET DISEASES OF FRUITS AND VEGETABLES 21 



The stem of a seedling may become infected at any point through- 

 out its length. The new lesions at first appear as brown specks 

 in the epidermis; later the spots enlarge lengthwise along the stem 

 and involve the tissues beneath the epidermis, finally causing pits 

 that may be deep and numerous enough to cause collapse of the 

 seedling. 



When the leaves are involved, the lesions usually start on the 

 under side of the veins and may lead to death of the leaf tissues around 

 the affected spots. These dry brown areas generally are small, ir- 

 regular, and elongated. When they are torn by the wind, the leaves 

 become ragged in appearance. Severe attack on the petiole may cause 

 dropping of the leaf blade, 



CAUSAL FACTORS 



The causal fungus, Collet otrichum lindemuthianum, may remain 

 viable through at least 2 years in seed harvested from diseased pods, 

 or may overwinter in the field on trash from diseased plants of the 

 previous season. The free spores when worked into the soil are not 

 viable after 8 to 10 weeks. The introduction of the disease into a 

 field occurs most frequently through the use of diseased seed. Under 

 favorable moisture and temperature conditions the fungus, which 

 has been lying dormant in the seed or soil, begins to grow and de- 

 velop spores. If the seedling is not killed before it emerges from 

 the ground, it may carry either the old or new spore-bearing lesions 

 above ground. Spores from either source can lead to new infections 

 when moisture conditions are favorable, so long as the tissues of the 

 pods have not passed the brittle or snap stage, For this reason 

 healthy-appearing pods picked during or immediately following rains 

 in diseased fields, or pods from healthy fields when packed with those 

 coming from diseased fields frequently develop spotting in transit. 



Growth of the fungus is possible throughout a temperature range 

 from 32° to 93° F., but most rapid development takes place at about 

 73°. Spores are produced abundantly on bean pods at 50° in 6 days, 

 but only sparingly at 82° and above. On moist, favorable bean tissue 

 spores can produce infections between 57° and 80°, the greatest 

 number being produced at about 71° to 73°. 



An abundance of moisture is essential for development and dis- 

 semination of the spores as well as for their germination and for 

 infection of the plant. Moisture may be present either as a film on 

 the plant surface or as vapor in the air. Humidities of about 95 

 percent at temperatures between 64° to 68° F. are favorable for 

 germination of the spores and penetration of the host. 



The incubation period ranges from 4% to 9 days, varying with the 

 temperature, the host, and the age of the tissues. Consequently, 

 pods that look sound when picked and packed may actually be in- 

 fected and develop lesions during transit and marketing. Young 

 succulent tissues are most easily infected. Pods are not subject to 

 infection after they pass the brittle or snap stage. 



Although no snap bean varieties are known to be immune to 

 attack by the fungus, there are all degrees of resistance. At one 

 extreme are those varieties on which only slight lesions are produced 

 on the youngest tissues ; at the other are those on which the lesions are 



