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When it comes to pests and diseases the adage “just when you think you’ve seen it all, something new comes along” couldn’t be more true. Two seasons ago a small patch in the middle of a broccoli planting started to look wilted and stressed even though we had just irrigated a few days before. The problem worsened to the point that it merited closer inspection. I waded out into the now stunted patch and pulled out a few plants to inspect their root system and what I found was shocking—swollen, distorted roots, some of which were laying above the soil surface.
After pulling out my IPM book from the University of California, it didn’t take long to discover that we had an outbreak of Clubroot—a soilborne, fungal pathogen that infects nearly all of the members of the Brassica family—broccoli, cauliflower, kale, collards and mustards just to name a few.
For the first two seasons it stayed pretty much confined to the same location so I figured we could just deal with it by not planting brassicas there, but this year that all changed. By mid-June, the disease had spread rapidly and nearly half of our fields were infected. The primary recommendation for dealing with the disease is to rotate out of brassicas for at least 3 years, but for us, this is practically impossible because nearly half of what we grow is in the brassica family and we plant 2-3 successions of them a year.
The other recommendation for dealing with clubroot is to maintain a soil pH of 7 or above—it only really thrives in acidic soils. This is where things get interesting. When we first arrived here most of the soils tested out with a pH of 7.2 or above. I knew that there was a general downward trend since we have farmed here, but I was really surprised when a recent round of tests revealed that in some of our fields the pH had dropped as low as 6.6. The reasons for this are complex, but the main culprit is the humic acid that is produced during the breakdown of all of the cover crops and compost that we have added over the years. Another contributing factor could be the leaching of calcium and magnesium during high rain years like the one we just had.
I received several encouraging responses when I asked about the problem on a local farmer’s forum from growers who had dealt with it by spreading lime to raise the pH and hadn’t seen the disease since. So yesterday I was out on the tractor spreading lime on our fields for the first time ever (we usually spread gypsum as a calcium source which is more or less pH neutral). Next year we will rotate out of brassicas on the infected fields and hope for the best.
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