California Fruit and Vegetable

Tag: UCCE Vegetable Crops Farm Advisor

  • Root Inhibitor Herbicide Injury in Tomatoes

    Pic 1 Swollen stems at the soil line

    Root inhibitor herbicides such as Trifluralin (Treflan HFP) and Pendimethalin (ProwlH2O) are soil applied and preplant- incorporated and used for control of many annual grasses and broadleaves in processing tomatoes in the Central Valley.

    This year, I have seen symptoms from these root inhibiting herbicide injury in a processing tomato field and the damage was quite uniform and widespread in the field. The affected plants had a very atypical swelling at the soil line, were wilting and collapsing (Pic 1, 2 and 3). The stems were very brittle and easily snapped at the soil line near the swollen stems (Pic 4), which is a characteristic of Pendimethalin injury. There was some lodging in the field probably due to winds or a rainy event (Pic 3).

    Pic 2 Wilted plants in the field.

    It is hard to predict why this happened as the herbicides are supposed to be incorporated into the top 2-3 inches of soil where the weeds germinate, and the transplants are plugged deeper. One of the hypotheses could be that there were some heavy rains that could have moved the herbicides deeper into the soil profile. Another possibility is that residual trifluranlin or pendimethalin from the previous crop along with current application may have caused soil herbicide levels to be beyond crop tolerance.

    Pic 3 A collapsed and dead plant in the field.

    Similar damage was observed in the Sacramento valley in 2021 where a young tomato field was affected significantly with root inhibiting herbicide injury. These are typical injury symptoms for these herbicides and can be seen on other veg crops such as melons, peppers etc.

    Please contact your local Vegetable crops Advisor if you see similar symptoms in the field. — By Jaspreet Sidhu, UCCE Vegetable Crops Farm Advisor

    Pic 4 A stem broken off at the soil line.

  • Fusarium Crown Rot in Watermelon

    In late June, I was contacted by a watermelon grower and visited his field near the border of Stanislaus and Merced County. In the field, 50% of the plants showed leaf and vine wilt and about 20% died (see the field image). When I made a closer examination, the color of leaf lesions were chocolate brown and stem appeared to be watery (see leaf and crown images). The grower told me that the field was just harvested once but symptoms were already present beforehand. The field was planted by a 45-ct and mini-watermelon cultivars.

    Vine-declined and wilted watermelon plants from the disease infection.

    An initial suspicion was a fusarium disease and/or a possible Gummy Stem Blight caused by Didymella bryoniae, though this is very uncommon for the watermelon in California. I sent leaf, runner, and stem samples to the UC Davis Fungal Pathogen Lab and the results came back with the Fusarium Crown Rot caused by F. solani f. sp. cucurbitae and possibility of F. falciforme. F. falciforme has not been described to infect watermelon but records showed that it could cause crown rot in muskmelon. Gummy stem blight (Didymella bryoniae) colonies were not recovered from the samples, which did not surprise me because this disease is mostly prevalent in the southeast and north area, such as Georgia, South/North Carolina, and Delaware.

    Unlike fusarium wilt caused by F. oxysporum f. sp. niveum, the vascular system of crown rot infected plants typically does not show discoloration far above soil line, instead, necrotic rot of crown and taproot can be seen. As the disease progresses, the rot on the crown develops from a light-colored, water-soaked area to be darker. Eventually, the entire plant wilts and dies. Evidence indicated that fusarium crown rot is more common on summer squash and pumpkin, however, all cucurbits can be infected.

    Water-soaked, necrotic rot of the crown

    Early planted fields can have a higher chance to be infected as disease is favorably developed under a cooler temperature. Soil moisture does affect the development of the disease. Extremely wet soil especially with drip tape breakage creates a favorable microclimate, which definitely accelerates the reproduction of spores and spread of the disease to other rows. Growers using surface drip irrigation should specifically pay more attention to the tape damage and fix the problem timely.

    Various types of information demonstrate that the pathogen (F. solani f. sp. cucurbitae) is seedborne and survives only for two to three years in soil. A four-year rotation of planting non-cucurbit species is usually chosen for the disease control. In addition, choosing clean seeds or fungicide-treated seeds can reduce disease initiation. More information about the Fusarium crown rot on cucurbits can be found at UC IPM: http://ipm.ucanr.edu/PMG/r116100911.html. – By Zheng Wang, UCCE Vegetable Crops Farm Advisor