An Apple Scab Primer For Primary Infections

Apple scab, caused by the fungus Venturia inaequalis, is a serious disease of apples in Ontario that affects foliage, blossoms and fruit. There are two types of apple scab infections. Primary infections occur on leaves and developing fruitlets and are caused by air-borne ascospores that overwinter on infected leaves on the orchard floor. Secondary infections are the result of infections by inoculum (conidia) produced on these lesions and can cause the spread of apple scab throughout an infected tree. If primary infections can be kept to a minimum, the need to control secondary infections is considerably relaxed by around the second cover spray. There are 4 requirements for an apple scab primary infection to take place: overwintering inoculum, susceptible tissue, the appropriate environmental conditions, and mature/discharged ascospores.

Overwintering Inoculum

Apple scab overwinters in lesions in leaf litter. Heavy rainfall in the fall of 2006 would have been conducive to late-season scab infections of leaves, after fungicide sprays had ceased. Therefore, even orchards that did not have fruit scab last year, may have developed late season leaf scab. A mild winter, followed by a wet spring, may result in higher survival of fungus and a higher inoculum pressure. Apple growers should be prepared for high scab pressure in orchards in 2007.

Orchards can be identified as "high" or "low" inoculum by determining Potential Ascospore Dose (PAD) in the fall after harvest. The procedure for this calculation is provided in Publication 360. It's too late to determine PAD for this season; however, if an orchard had fruit scab or severe leaf scab in 2006, it is definitely a "high" inoculum orchard.

Susceptible tissue

Scab infections can occur as soon as green tissue is present on apple trees. Leaves are most susceptible to infection before they are fully expanded. Once they are fully expanded (5-8 days after emergence), they are highly resistant to infection. However, since new leaves are produced in fairly rapid succession, it is necessary to make repeated applications of fungicides in order to maintain control of scab on newly emerging susceptible leaves. Fruit remain susceptible until harvest, but the duration of the wetness period required for infection increases with the age of the fruit.

It is always easier to prevent apple scab infections than it is to manage them -- so applying a fungicide at green tip to ½ inch green is an excellent preventative tool. The first fungicide must be applied before the first scab infection period after bud break. Otherwise, you may be playing catch-up for the rest of the season. An early season copper spray, applied for fire blight control, can count as the first scab fungicide.

Environmental conditions

The third component to an apple scab infection is the presence of the appropriate weather conditions (temperature and moisture) conducive to the development of apple scab. The Mills model (Table 1.0) has long been used to predict the potential for apple scab infections. This table is based on the fact that the apple scab fungus grows best at temperatures between 16º and 24ºC, and so requires the shortest period of wetness for infection to occur in that temperature range. At temperatures above or below this range, tissues must be wet for longer periods in order for infection to occur. In orchards with low scab pressure the length of wetting period should be counted during the day (8 am to 7 pm) from the first hour the rain was recorded until the leaves are dry (do not include overnight rain in calculations). However, in high inoculum orchards, the length of wetness should be calculated from the start of the rain until the leaves are dry regardless of the time of day (ie. include nighttime wetting periods). It is the duration of the event that is the important component of determining whether an infection period occurred. Intermittent periods of drying (up to 8 hrs dryness between wetting periods) should be added together to determine the total length of an infection period. Wetting periods can also be caused by dew formation after low night temperatures.

Table 1.0 Revised Mills table from Cornell University (http://www.nysaes.cornell.edu/pp/extension/tfabp/mills.shtml)

¹Average temperature during the wetting period in Fahrenheit or Celsius
²Minimum wetting period needed to achieve infection
³Time in days until visible symptoms appear

Mature Ascospores

The fourth component of apple scab primary infections is the presence of mature ascospores. These ascospores develop in infected overwintering leaves at bud break. The greater the scab pressure last year, the greater the number of ascospores that will be present in overwintering leaves this spring. The rate at which the ascospores are discharged is determined by temperature: the higher the temperature, the greater the amount of ascospores that are discharged While the ascospore concentration in orchard air commonly peaks between pink and petal fall, it is the early infections that can occur as early as green tip that will initiate a scab epidemic. As a result it is critical that scab programs be started before the initial infection period occurs around green tip, if protective fungicides are used and the orchard was not super clean at the end of the previous season. In orchards that were very clean last year the total number of ascospores shot into the air will be small and the likelihood that these few ascospores will find a green tip will also be very small. Under such optimal conditions a green tip spray can be omitted. However, most orchards do not have such optimal conditions, and in orchards where scab is present-delaying an early season scab spray can result in an early season scab infection that can be a nightmare to manage.

Being aware of conditions required for primary infection periods can help growers prevent apple scab infections. Apple scab is best managed preventatively, and there are few viable options for growers who are trying to deactivate scab lesions. The cost of an early-season protectively applied spray is a small price to pay for the peace of mind that you are doing your utmost to prevent an epidemic.

So what are the best strategies for managing apple scab? Stay tuned for our next article on scab management programs.

Learn more

• Scab Management Fungicide Programs