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The recent ominous specter of declining soybean yields for North American producers for the next decade is being strengthened by continued government reports of Asian soybean rust in Florida, the Gulf Coast States and Texas. The spores for this plant pathogen arrived on the winds of the numerous hurricanes this season. In figure 1 we see the path of Ivan, the most widespread storm as it grazed the already infected northern growing regions of South America (pink zone). In that southern passage, Ivan picked up spores of the fungus, before turning north into the regions in the United States that could provide a safe haven for the spores over the winter (pink tracking line). Those regions are, Florida, the Gulf Coast and Texas, along the western side of the Gulf of Mexico. In fact, Ivan actually took a looping turn over the southeast in order to track backwards across the Gulf of Mexico into Texas. The path of this hurricane effectively spread spores across the very areas of the continent that can provide a perfect winter shelter for the cold sensitive spores. All that is needed now to infect crops to the north this coming growing season is a combination of moderate temperatures and either regular rains or strong dew formation on the soybean crops in the Corn Belt during the pod setting and filling stage in August and September.
The ideal temperature for the blooming of the fungus is between 65°F and 78°F. Above that temperature spread, the rust spores can't take the heat and below that temperature spread they can't take the cold. The 100% stress free condition for the rust is to have temperatures between 65°F and 78°F with regular rains or abundant dew. These temperatures are on the cool side for the Corn Belt summer. In China, where the growing conditions resemble the conditions in the Corn Belt, there is often a yearly dieback of the rust during the warm times in the summer, when the temperatures are above the optimal for its survival. If, during a warm trend, there is a brief cool down so that dew can start forming again, the fungus begins thriving. In general, the rust spreads most easily when the rain and temperatures are in these temperate zones. In China, this most often happens in the late summer and early fall.
This is worrisome to the growers in the United States since the ideal growing weather that produced the record soybean crop in this current year of 2004 would be a disaster if the rust spores were present in the fields in the Central States and northern sections of the Ohio and Mississippi valleys. A year like this past growing season (2004) would decimate the crop next year, when this year it produced the second greatest harvest on record. To be widespread enough to do this kind of damage, the spores would have to be broadcast to the north from the south. The southern areas are where the spores can survive the winter.
In figure 2 we see the areas where the spores can best survive. The best is the Gulf Coast of Texas optimally and as a secondary site the Gulf Coast of Florida (pink areas). Texas is optimal because when cold temperatures drop down into Florida from the north during the winter, they threaten the viability of the spores in that area. It is more likely that the freezing temperatures needed to kill the spores can happen in Florida than it is for freezing temperatures to sustain themselves in southern Gulf Coast Texas. Generally the spores will tend to stay in these more tropical areas until a Gulf Coast monsoon provides the northward blowing winds needed for their dispersal (red arrow). A monsoon coming from the western Gulf Coast would most likely place spores in Iowa as the primary northern latitude state for the emergence of any future epidemics. This is due to the prevailing clockwise winds around a stable high- pressure area over the southeast (green area). High-pressure over the southeast during the late summer and early fall would provide the perfect environment for the spread and growth of the Asian Soybean Rust into the Corn Belt.
Given the above, it appears to Doc Weather that Asian Rust will be a pretty strong player in next year's soybean outlook for the United States. This is because the eclipse pattern is perfect for the possibility of a sustained Gulf of Mexico monsoon in the late summer and early fall in 2005. The monsoon should emerge in two key timeframes. One is from the last week of June to the third week of July. A good monsoon pattern similar to this year's pattern, with regular rains and cooler temps to the north looks likely due to the action of the lunar node on the eastern pair of eclipse points. This pattern is illustrated in figure 2. This early monsoon should fade for the first two weeks of August and then pick up again from mid August until mid September as the node once again becomes active. As already mentioned, the American continent has a pattern that is similar to China where the dominant infestations happen in the fall rather than the spring, due to the inability of the fungus to survive the heat of the high summer. That being the case, it looks very probable that next August and September are likely to provide good growing conditions for beans setting or filling pods, and simultaneously the cooler than normal, moist conditions will also provide optimal conditions for the growth of the fungus in the Ohio and Mississippi Valleys.
Regarding the most likely place for the emergence of the infestation, this winter the probability of strong cold coming into Florida is very good due to the placement of a dominant 45° jet curve (figure 3) from the eastern pair of eclipse points (red curve). This jet curve runs up the East Coast and will most likely steer cold fronts coming across the country towards a deep track into the southeast. The cold pattern (blue arrow) will probably not unfold until the current moderate el Nino event is finished in February. After that time cold temperatures in the southeast will most likely kill many spores of the rust.
In conjunction with this, the recent floods in Texas and the el Nino wetness pattern behind these floods should support the survival of many spores in the fields in southern Texas this spring. Doc Weather's bet is that a high-pressure area should center over Georgia in July and late August producing a warm and wet clockwise circulation around the high. This should push a plume of warm wet air through Texas northward into Iowa sometime in the middle of July and again in late August 2005. Ironically, this will most likely be both good for the beans and bad for the beans. Stay tuned to Doc Weather for future updates on this impending weather driven threat.