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Recently, there has been an interest in pecan scab control based on leaf wetness following rain (HortTechnology 5:225-230; Proc. Southeastern Pecan Growers Assn. 89:109-153). With regard to scab and extended leaf wetness following rainfall, J. R. Large published an interesting note in the 1957 Proc. Southeastern Pecan Growers Assn. Mr. Large's paper, "Field records indicate that pecan scab is more serious after evening or night rains than it is following morning and afternoon showers," is reprinted here as follows:

"Pecan scab was very serious in the experimental plots near Monticello, Florida in 1956, but the disease was not as severe in the experimental plots near Quincy, Florida. This was indicated by general observation of scab development in the two orchards and also by the nut scab counts which were made on August 9 at Monticello, and on August 10 at Quincy, Florida.

The August 9 scab count on Moore nuts sprayed with regular Bordeaux mixture indicated there were 72 percent marketable nuts on the trees at Monticello and 95 percent on August 10 at Quincy, Florida. On these dates, the nut scab count on Moore pecans sprayed with Zineb (Dithane Z78) plus Volk summer oil indicated there were 87 percent marketable nuts at Monticello and 95 percent at Quincy. On unsprayed trees there were 15 percent marketable nuts at Monticello and 50 percent at Quincy. These differences are believed to have been due to climatic conditions. In 1956 there were 37 rains at Monticello during the six month growing season, April 1 to September 30. At Quincy there were 75 rains during the 6 spring and summer months.

Very little scab developed during April and May, but infection tended to increase during June, July and August. Two factors contribute to this increase in disease susceptibility later in the season. First was an increase in rainfall during these months. The second was due to rapid increase in size of the nuts, at this time of year the young growing tissue is very susceptible to infection by the scab fungus.

At Monticello there were 9 rains, 9.12 inches in June, 11 rains, 5.60 inches in July and 5 rains, 3.24 inches in August. A total of 25 rains and 17.96 inches of water. The record at Quincy was 18 rains, 7.21 inches in June, 21 rains, 8.23 inches in July and 14 rains, 1.69 inches in August. A total of 53 rains and 17.13 inches of water. Even though there were twice as many rains at Quincy as occurred at Monticello, pecan scab was more serious at Monticello than it was at Quincy. However, by studying the weather records it was observed that in 1956 the time of day that the rains fell determined their effect on pecan scab development. If the rains fell before 6 PM the nuts dried off before sundown and were much less susceptible to scab infection. When the rains fell after 6 PM the pecan nuts remained wet all night. This was an ideal condition for the growth and infection by spores of the scab fungus. The reason that pecan scab was so bad at Monticello was that of the 25 rains that occurred in June, July and August, 15 fell after 6 PM, so the nuts remained wet for 15 nights or 180 hours. At Quincy although 53 showers occurred during these three months only 7 fell after 6 PM. The nuts remained wet for 7 nights or 84 hours. This indicates that there were 96 more hours when the leaves remained wet at night at Monticello with ideal conditions for infection and growth of the pecan scab fungus".

Table 1. Comparison of scab control on Moore pecans at Monticello and Quincy, Florida in 1956.
Marketable nuts - %

Table 2. Rainfall records for six spring and summer months for 1956 at Quincy and Monticello, Florida

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