L.P.Berkett, IPM Specialist

September 1, 1998

The Dreaded "Pin-Point" Scab:

This season has been one of the worst seasons for scab for some orchardists. Unfortunately, it is not over yet. "Pin-Point" or "Storage" scab can develop under certain conditions and ruin a seemingly non-infected crop in storage. Dr. Dave Rosenberger of Cornell's Hudson Valley Lab pulled together the following information for an article that appeared in Scaffolds on Aug. 25, 1998, about (1) the conditions under which storage scab poses a threat and (2) what you can do to prevent or minimize a problem:

"Apple scab that develops on fruit after harvest is known as "storage scab" or "pin-point scab". The latter name derives from the fact that scab lesions developing during storage are often quite small and can appear as black "pin-points" on the apple skin. The 1998 apple crop is at a greater-than-usual risk for storage scab because apple scab was prevalent in many Northeastern orchards throughout spring and summer of 1998.

Storage scab poses an economic threat only when both of the following conditions are met:

1. Scab inoculum is abundant in the orchard prior to harvest.

2. Fruit in the orchard are exposed to a continuous wetting period of at least 48 hours duration at a time when fungicide residues from the last spray have been depleted.

"Abundant inoculum" is a very subjective criteria. Orchards with a few old lesions from infections that occurred in May will not have enough inoculum to initiate storage scab. However, this year some orchards had extensive infections during May that resulted in two to four infected leaves per terminal. These infections were frequently arrested by using applications of SI fungicides plus captan. In many cases, the scab epidemic was contained, but enough inoculum has persisted to allow new infections to develop on leaves during late summer.

The best way to determine if orchards have active inoculum is to check the youngest terminal leaves, especially on water sprouts near the upper-center part of the canopy. Where scab has remained active through summer, these youngest terminal leaves are now showing active scab lesions on the upper and/or lower surfaces. Lesions on the lower surface of the leaves vary from pale, diffuse brown spots to very dark-colored brownish-black spots. Scab lesions usually have a fuzzy or velvety appearance with lesion margins that are somewhat indistinct.

Some orchards may have an abundance of other leaf spots that are of no concern at this time of year. Scab lesions can be distinguished from leaf spots caused by other fungi because the latter usually appear as round lesions with distinct margins and necrotic tissue that is visible on both the upper and lower leaf surfaces. Mites, including rust mites, can also cause the entire under-surface of leaves to appear brown or bronzed in some orchards. Mite damage on the under-side of leaves should not be confused with the more distinctive spots caused by scab.

Continuous wetting periods of at least 48 hours are required to initiate infections on fruit during the preharvest interval. Fruit gradually become more resistant to apple scab as they mature. With extremely high inoculum levels, a few fruit infections might occur with wetting periods as short as 30 hours, but economic damage is not likely unless wetting periods exceed 48 hours. Very severe infections could be expected if we should encounter continuous wetting periods of more than 96 hours. Drying periods as short as two hours in the middle of longer wetting periods will significantly reduce the amount of infection that occurs.

Infections occurring during the last week (and perhaps two weeks) prior to harvest may pose less threat than infections that occur slightly earlier during the preharvest interval. Apparently infections occurring the during the last week before harvest are not sufficiently well-established to allow further development during cold storage. However, delays in cooling fruit after harvest could allow even those "last-week" infections to develop symptoms during storage.

Wetting periods required for storage scab have been investigated in various production regions around the world. Experiments were conducted with Golden Delicious, Granny Smith, Starking Delicious, Starkrimson Delicious, and White Winter Permain. These cultivars showed no detectable differences in susceptibility. This data suggests that differences in susceptibility of mature fruit are probably minimal if fruit are exposed to similar inoculum levels. Under field conditions, however, susceptible varieties like McIntosh are more likely to have high levels of inoculum than are more resistant cultivars like Delicious because the higher susceptibility of McIntosh foliage increases the likelihood that leaves will carry sufficient inoculum for generating storage scab.

The probability of getting weather conditions that favor severe storage scab are relatively low because the probability of having continuous wetting for 4 days during harvest is low. Furthermore, enough rain would be needed prior to the four-day wetting to remove all captan residues from the fruit. Should such conditions develop, however, losses could be very high in orchards with abundant inoculum. Therefore, an additional spray of captan may be necessary in high-inoculum orchards if weather predictionscall for extended wetting periods.

If high-inoculum orchards are left unprotected through an extended wetting period, nothing can be done to stop symptom development after the fruit infections are initiated. Postharvest drenches are not effective for controlling storage scab. Field applications of benzimidazole fungicides immediately following a late-season infection period will be of no value because most strains of apple scab that persisted through summer are resistant to the benzimidazoles.

If weather conditions favor late-season scab development and high-inoculum blocks are not protected with fungicide, then the best solution will be to sell the potentially-affected fruit as soon as possible after harvest. In experiments conducted in South Africa, the first symptoms of storage scab appeared on Granny Smith after 80 days at 1-2 C as compared to 35-45 days for fruit at 20 C. However, scab lesions might appear in less than 80 days at 1-2 C on fruit that were infected earlier during the preharvest interval. Other studies have shown that new lesions appear more quickly at higher temperatures, but the total number of lesions is ultimately the same if apples are stored long enough to allow symptoms to develop at cold temperatures. Storing apples under reduced relative humidity can minimize lesion size because lesions develop greater size when fruit are held under high-humidity conditions, especially if the latter results in condensation developing on the fruit surface during storage.

The bottom line: Risks of storage scab are relatively small even in a year when inoculum is relatively abundant. However, the consequences of storage scab can be severe since infection rates can reach 100% and the losses become evident only after storage costs have been incurred. Therefore, monitor orchards now to determine if scab is active. Follow weather forecasts carefully. Apply captan in high-risk blocks ahead of predicted extended-wetting periods."

 Good Luck.... this will be a good year to use one or more of the sanitation practices to reduce overwintering inoculum once the harvest is complete and the leaves are on the orchard floor.

L.P. Berkett

____________________________________________________________________________________________________

Where trade names or commercial products are used for identification, no discrimination is intended and no endorsement is implied. Always read the label before using any pesticide. The label is the legal document for the product use. Disregard any information in this newsletter if it is in conflict with the label.