Apple Scab Model

Model description

The fungus causing apple scab overwinters as ascospores in old leaves on the ground. It can also overwinter as mycelium and conidia in shoots or buds, bud this is normally not as important. Ascospores are discharged during rain, and can infect new leaves, shoots and fruits when conditions are favourable. Apple scab disease warnings is primarily based on two models, where one deals with conditions for infection. The other model covers the maturation and discharge of spores from old leaves on the ground and shows how many spores that are discharged at different times of the season. The parameters used in apple scab warning is hourly values of temperature (Tm/Tmf), precipitation (RR) and leaf wetness (Bt/Btf/Btff).

Infection model

Ascospores need wet leaves, shoots or fruits for a certain duration for the fungus to infect, and how long it takes is dependent on temperature. Leaf wetness sensors are used to indicate how long surface wetness is present. The infection table, often called Mill’s table, shows how many hours it takes before the first spores infect at different temperatures. At the optimal temperature, 16 - 22 °C, it will take 6 hours.

Risk of infection is expressed as an hourly value greater than zero. The risk is accumulated, and increases for each hour with leaf wetness. When the risk is above 90, the risk is set as yellow (possible danger). When the accumulated risk exceeds 100, symbol turns red as a sign of infection risk. If there has been no leaf wetness for 8 hours, the accumulated risk is reset. If the number of dry hours is less than 8, the accumulation will start again without resetting the risk value.

Spore maturation model

A model for ascospore maturation that was developed in the USA has been modified to work under Norwegian climatic conditions. Maturation is a function of accumulated heat sum (degree-days, base 0°C) from a certain starting date (when first spores are mature) and calculates the period when there is a risk of ascospores being discharged. Protracted dry periods without precipitation or long periods of dew (more than 7 days) will stop the degree-day accumulation. The degree-day accumulation will start again as soon as precipitation has been registered. Several or long dry periods without rain can prolong the season where ascospores are released and can infect.

RIMpro

In addition, VIPS also presents apple scab warnings from RIMpro, which is developed and produced by Marc Trapman, Bio Fruit Advies, The Netherlands. This is an external service, but is based on the same weather data as all other models in VIPS.

Warning interpretation

• Green: It has not been conditions favourable for apple scab infection today
• Yellow: Apple scab warning is probable if it stays humid/wet for the next hours
• Red: It has been conditions favourable for apple scab infection today

Illustration 1. Screenshot

The blue line shows spore maturation, and when it reaches 100% the primary season is over and no more warnings will be issued. The black line is accumulated infection risk, and when it reaches 90 or 100 the background turns yellow or red, respectively.

Table 1. Infection risk based on disease pressure and cultivar.

	Cultivar resistance
Scab attack last year	High	Medium	Low
Low	Low	Low	Medium
Medium	Low	Medium	High
High	Medium	High	Very high

Time of start and end of the warnings

Start: Green tip in early flowering cultivars (for instance Gravenstein or Summerred)

End. After primary season is over (when ascospore maturation has reached 100%)

Testing and validation of the models

Nationally

Almost every year since 1989 has the ascospore discharge of apple scab been monitored by spore traps in an apple orchard at Ås. For some years registrations has also been made in other locations. The spore trap data shows when spore discharge start in the spring, what weather conditions they are released under and when the primary season is over. A model for spore maturation published in the USA (Gadoury og MacHardy, 1982)has been validated by spore trap data from Ås, and an adjusted version of the model adapted to Norwegian climatic conditions has been published (Stensvand et al. 2005). Later, several models for ascospore maturation and dissemination by pear scab (Venturia pyrina) investigated using spore trap data from Norway, Belgium and Australia (Eikemo et al. 2011), and the model performing best was the one we now use for apple scab. An important challenge with pear scab is that survival of mycelium and conidia in shoots (wood scab) is much more important than for apple scab. Conidia from wood scab can be a source of inoculum even after the ascospore source in the old leaves on the ground is depleted.

Internasjonalt

RIMpro is one of the most commonly used tools for warnings against apple scab. It is based on the literature described above, and is validated and updated annually based on new research.

References

Eikemo, H., Gadoury, D.M., Spotts, R.A., Villalta, O., Creemers, P., Seem, R.C. & Stensvand, A. 2011. Evaluation of six models to estimate ascospore maturation in Venturia pyrina. Plant Disease 95:279-28

Gadoury, D. M., and MacHardy, W. E. 1982. A model to estimate the maturity of ascospores of Venturia inaequalis. Phytopathology 72:901-904.4.

Stensvand, A., Eikemo, H., Gadoury, D.M. & Seem, R.C. 2005. Use of a rainfall frequency threshold to adjust a degree-day model of ascospore maturity of Venturia inaequalis. Plant Disease 89):198-202.

Contact:

Håvard Eikemo - haavard.eikemo@nibio.no

Arne Stensvand - arne.stensvand@nibio.no

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