Peach Leaf Curl Epidemiology: Abiotic Factors

Earlier this month we discussed peach leaf curl biology and control. In this issue we’ll explore how the abiotic environment influences infection by the leaf curl fungal pathogen, Taphrina deformans.

Environmental effects.  Leaf curl development is dependent on two abiotic environmental factors: moisture and temperature. At near optimum temperatures, infection begins after a minimum of 12 hours wetness and increases steadily until 48 hours of wetness. Longer durations of wetness do not increase disease levels. The duration of surface wetness is the primary moisture determinant for infection. Thus, if the wetness period was caused by rainfall, the amount of precipitation does not influence the severity of infection. Conversely, wetness periods from dew or fog, unless accompanied by rain, are often too short for infection.

Air temperature during the wetness period needs to be less than 61°F for foliar infection to occur. The amount of infection increases as temperatures decrease to 41°F. So, clearly, T. deformans favors cooler temperatures over warmer temperatures. Unfortunately, no data are available for temperatures below 41°F, so it’s possible that even greater amounts of infection occur below this threshold.

A tale of two years.  In 2012, leaf curl development in our research Redgold nectarine block at the RAREC was all but absent. Only 0.4% of shoot buds became infected on non-treated control trees. In comparison, non-treated trees in the same block in 2004 sustained 39.3% bud infection. In both cases, the block received no fungicide sprays during the entire prior years, 2011 and 2003, respectively.

What caused the difference in infection between the two years?  Certainly, inoculum levels could have been different. Another possible explanation was the difference in March weather conditions, a key month during which leaf curl “takes hold”. In the table below are listed March temperature and rainfall data for each year along with the 30-year normal. Based on the research results presented above, we can assume that normal temperatures favor leaf curl development; a normal average temperature of 43°F is clearly ideal for T. deformans.

In disease-unfavorable 2012, the daily average, minimum, and maximum temperatures were 7.0, 5.6, and 8.7 °F warmer than normal, respectively (Table). Conversely, in disease-favorable 2004, these temperatures were nearly the same as the normal temperatures, being only 1.2 to 1.5 °F higher. In addition, total rainfall in March 2004 was more than double rainfall in March 2012, indicating a greater opportunity for more frequent and/or longer wetness periods. Indeed, twice as many rain events having greater than 0.25 inches of rainfall occurred in 2004 than in 2012.

Given the above comparison, normal March temperatures and rainfall translates into favorable conditions for infection, assuming inoculum is present. If an unusually warm and/or dry March occurs, perhaps attributable to climate change, leaf curl infection is unlikely.

March 2013. So, relative to leaf curl infection, how are weather conditions so far this month? As of midnight March 11^th, temperatures have been slightly below normal.  For the first 11 days of March, the average, minimum, and maximum daily temperatures were 38, 30, and 47°F, respectively, which are only 1.6 to 2.6 °F below normal for the first 11 days. Given the preference of T. deformans for cooler conditions, the temperatures so far this month have been favorable for infection.

As I write this article on 12March, a cool front is depositing moderate to heavy rain. High and low temperatures are forecasted at 57°F and 34°F; a resultant mean temperature of 46°F would be favorable for infection, and the rainfall is producing a wetness period much greater than the minimum 12 hours. However, shoot buds are currently holding tight with little, if any, bud swell or loosening of the bud scales. So, an infection period from this weather front is unlikely given the current tree phenology.

The extended forecast for the remainder of this week and into next week shows highs at 45-55°F and lows at 25-35°F. Given these favorable temperatures and continued development towards bud-break, it’s likely that the next rainfall will be the first leaf curl infection of the 2013 season.