Basil plant infected with downy mildew.

Fig. 1 Basil plant infected with downy mildew.

Updated: November 5, 2021
By Gerald (Jerry) Brust , and Karen Rane

Basil Downy Mildew Cultivar Study

Summary: This study examined five cultivars (Passion, Obsession, Devotion, Thunderstruck and Prospera) of basil that are reported to have resistance to basil downy mildew with a susceptible cultivar (Aroma). A field trial was planted in July and harvested throughout the summer and fall. Four of the five cultivars (Passion, Obsession, Devotion and Thunderstruck) became infected with downy mildew at the same time as did the susceptible cultivar (Aroma). In this study only Prospera did not develop downy mildew symptoms. The infected cultivars had significantly reduced marketable yields at first and then no marketable yields 4 weeks after infection. Prospera continued to have good marketable yields throughout the entire study period.

Introduction: Downy mildew of basil is a devastating disease that has plagued growers of the herb for years. It was first reported in the United States in 2007 and has since spread widely to wherever basil is grown. Basil downy mildew is caused by Peronospora belbahrii, a fungus-like microorganism in the group called water molds. This disease affects the leaves, branches, and stems of sweet basil with green-leafed varieties being particularly susceptible. Basil that has become infected with downy mildew has a yellowish appearance that is similar to a nutritional problem. Yellowing of leaves normally first appears on lower areas of the plant (fig. 1).

Gray sporulation develops on the underside of the yellow areas of leaves (fig. 2). As the disease advances, the leaves turn completely yellow and fall off, the stems wither, and the plant eventually dies. Trial: Seedlings of Obsession, Passion, Devotion, Thunderstruck (Rutgers breeding program), Aroma and Prospera were obtained from a commercial herb producer and transplanted at CMREC-UMF near Upper Marlboro, MD on 21 July.

Fig. 2 Dark ‘fuzzy areas’ (spores) on underside of basil leaf infected with downy mildew.
Fig. 2 Dark ‘fuzzy areas’ (spores) on underside of basil leaf infected with downy mildew.

The transplants were planted into two rows of white plastic with drip irrigation. There were 4 reps of each cultivar with 6 plants per rep. Plants were set 2 ft apart in a row and rows were on 6-ft centers. First harvest occurred on 4 August for all cultivars except Obsession. Harvest consisted of taking the top part of plant and cutting it 4-8 inches from the soil line. This cutting was inspected for leaf and stem quality and any blemished leaves or stems were removed and the rest of the harvested material was placed in a plastic bag marked and placed in a cooler. Bags were then taken to the lab where the harvested basil was examined again for quality and recorded as marketable weight per plant. Harvests occurred approximately every two weeks (4, 17, 31 August; 15, 28 September and 15 October). Marketable weights of the basil cultivars were subjected to an ANOVA and significant results (p< 0.05) were then analyzed using Tukey’s HSD means separation test with significant differences of p< 0.05 being reported.

Results: Basil plants grew well and after two weeks after transplanting were ready for the first harvest except for Obsession, which grew more slowly. The first harvest (4 August) was small and will not be shown here. No symptoms of downy mildew were observed on any basil cultivar during the first harvest. Second harvest (17 August) was good with the cultivars Passion, Devotion and Aroma yielding well with Passion having significantly greater (p<0.05) marketable yields than Prospera and Obsession (fig. 3). Fusarium wilt of basil was found on a few plants of Thunderstruck at this time. No downy mildew was found on any cultivar on this harvest date. The third harvest (31 August) showed that all basil cultivars yielded very similarly with an average of ~100g/plant (fig. 4). The Fusarium wilt on Thunderstruck did not spread and apparently was confined to the 3 plants that had been removed. No downy mildew was observed on this harvest date.

On the 4th harvest (15 September) downy mildew symptoms and signs were found on Passion, Obsession, Devotion, Thunderstruck and Aroma, but not on Prospera. Marketable yields were reduced significantly (p<0.05) by 55% for Passion, Obsession, Devotion and Thunderstruck compared with Prospera (fig. 5). The susceptible cultivar Aroma had its marketable yield reduced by 95% compared with Prospera and by 85% when compared with Passion, Obsession, Devotion and Thunderstruck. Prospera did not show any symptoms of basil downy mildew on this harvest date. By the 5th harvest (28 September) only Prospera had any marketable yields (fig. 6). Prospera showed no symptoms of basil downy mildew. The cultivar Aroma was defoliated on this harvest date (fig. 7) and while the other cultivars still retained much of their foliage, that foliage showed symptoms of basil downy mildew infection making them unmarketable (fig. 8).

Second harvest of basil.
Fig. 3 Second harvest of basil.
Third harvest of basil.
Fig. 4 Third harvest of basil.
Third harvest of basil.
Fig. 5 Fourth harvest of basil.
Fifth harvest of basil.
Fig. 6 Fifth harvest of basil.
Aroma cultivar (on right) next to Prospera cultivar on left 28 Sept 2021.
Fig. 7 Aroma cultivar (on right) next to Prospera cultivar on left 28 Sept 2021.
Resistant cultivar of basil with downy mildew infected leaves making them unmarketable.
Fig.8 Resistant cultivar of basil with downy mildew infected leaves making them unmarketable.

Discussion: All cultivars yielded well after the 1st harvest. It wasn’t until downy mildew disease was found in most of the basil cultivars by the fourth harvest that -significant differences in yields between those cultivars were observed - The four basil cultivars that were bred for resistance to downy mildew (Passion, Obsession, Devotion and Thunderstruck) showed downy mildew symptoms and signs by mid-September as did Aroma, which does not have resistance. Prospera showed no signs or symptoms of downy mildew throughout the trial. Aroma became completely defoliated by the fourth harvest, while the four infected resistant cultivars were still producing marketable yields albeit significantly less than Prospera. By the 5th and 6th harvests four of the resistant cultivars produced almost no marketable yield. Prospera continued to produce marketable yields through the entire study. At the 4th harvest many infected leaves had to be discarded from the infected resistant cultivars before a marketable product could be produced. Cleaning plants of infected leaves would increase production time and costs and be impractical for growers.

Print Article

Return to Horticulture and Disease Topics

This article appears on November 1, 2021, Volume 12, Issue 7 of the Vegetable and Fruit News  

Vegetable and Fruit News, November 2021, Vol. 12, Issue 7

Vegetable and Fruit News is a statewide publication for the commercial vegetable and fruit industries and is published monthly during the growing season (April through October). Subscribers will receive an email with the latest edition.

Subscribe