366 



R. T. BINGHAM 



Some improvement was obtained by: using inner chambers of poly- 

 ethylene film; protecting outer rows of seedlings with paper or wet burlap 

 barriers to prevent drying; maintaining standing water droplets on seed- 

 lings by intermittent, hand misting (Fig. 5); and using R. hudsonianum 

 leaf rather than R. visoossimum bush inoculum. Despite these improvements, 

 given a really large test involving several contiguous chambers and a 

 warm inoculation season, fluctuations in levels of infection within and 

 between the chambers have remained large. For instance, in 1966 when 800 

 running feet of nursery beds containing more than 75,000 seedlings were 

 inoculated in three large, contiguous chambers (Fig. 1), it required almost 

 8 hours for a 6-man crew to lay out infected R. hudsonianwn leaves on 

 screens suspended above the seedbeds (Fig. 6) . The first of three chambers 

 on the east was "buttoned up" at 10:00 a.m. and the last on the west at 

 3:00 p.m., by which time some of the succulent R. hudsonianum leaves were 

 wilting. During this inoculation ambient air temperatures reached 95°F, 

 inner chamber temperatures 87°F. On the warmest afternoons a 3-man crew 

 simply could not move fast enough with hand-mist nozzles to maintain water 

 droplets on all seedlings. This is not surprising when one considers that 

 at 50°F a cubic meter of saturated (100% R.H.) air contains 9-1/2 gm of 

 water, while at 86° F it contains 30-1/2 gm--or 3-1/4 times as much. 

 Despite these problems, 3 years after this single, 72-hour inoculation, 

 average level of infection in 10 "non-resistant" control lots of the main 

 experiment was 88% (Table 1) . 



Figure 6. Infected Ribes 

 hudsonianum leaves being laid 

 out on inverted nursery bed 

 cover screens, approximately 

 18 inches above pine seed- 

 lings. Leaves from five or 

 more different Ribes bushes 

 are systematically mixed 

 during this process. 



