No. 13 — Aug. 31, 2020


Norton and Disease: Resistance, Tolerance and Susceptibility

The Norton grapevine is less susceptible to the common grape pathogens compared to many of the other hybrid grape cultivars or American cultivars. Common grape pathogens include black rot, downy mildew, powdery mildew, Phomopsis, and anthracnose. Just because Norton is less susceptible to these common grape pathogens does not mean that Norton does not need to be protected with fungicides. It is important to remember the disease triangle-susceptible host, -pathogen, and -environment. If the environment is or becomes conducive for the pathogen and a susceptible host is present then the propensity for infection increases and the onset of disease will commence. Besides the common pathogens above, Norton is also susceptible to a number of trunk diseases.

A field survey of grapevines showing characteristic dieback resulting from trunk disease determined that Norton grapevines were infected with nine different species of trunk diseases (Table 1) (Jose Ramon Urbez-Torres et al. 2012). The survey was conducted in 2006 and 2009 in 30 Missouri vineyards. The most prevalent (27% of samples) fungal species isolated from symptomatic wood (wedge shaped canker or black wood streaking) was Neofusicoccum ribis. The second most prevalent fungal species isolated was Pestalotiopsis sp. at 18%. Pathogenicity studies showed that Norton was susceptible to 14 different species of fungal trunk disease. Neofusicoccum ribis was the one of the most aggressive pathogens as determined in pathogenicity trials in Norton. In pathogenicity trials, N. ribis vascular discoloration was on average 69.5±7.1 mm in length compared to Pestalotiopsis sp. discoloration was on average 12.3±2.6 mm in length six months after inoculation. In addition to being a trunk disease, N. ribis can also cause fruit rot.

Macrophoma rot over the years has been attributed to the Botryosphaeria complex. Recent research out of Virginia as well research on Muscadine grapes has attributed Macrophoma rot being caused by N. ribis. In a survey of seven Petite manseng vineyards in Virginia, Macrophoma rot was prevalent in all vineyards surveyed. Further research tried to determine the phenological stage when N. ribis begins to cause infection.

Table 1. Fungal trunk disease pathogens isolated from symptomatic Norton grapevines in Missouri in a survey published in 2012. See Urbez-Torres in references.
Botryosphaeria dothida
Diplodia seriata
Neofusicoccum ribis
Neofusicoccum vitifusiforme
Pestolotiopsis sp.
Pestolotiopsis uvicola
Phaeomoniella chlamydospora
Phomopsis viticola
Togninia minima

The results were inconclusive, but suggest that infection can occur from flowering to two weeks post-veraison. One of the more interesting results from the Virginia research was the impact of sun exposure on disease incidence and severity. Clusters of Petite manseng in full sun had greater disease severity and disease incidence than shaded clusters within the canopy. The initial onset of Macrophoma rot symptoms on grape berries very closely resembles Phomopsis lesions and sun burn of grape berries. As the disease progresses and berries shrivel and produce pycnidia, the symptoms closely resemble advanced stages of black rot.

Over the last few years, a number of growers are reporting that Norton berries are shriveling up prior to physiological maturity. In most cases, these symptoms are observed on a few clusters on a vine. Often grape clusters on one shoot will be shriveled but the clusters on the adjacent shoot will be normal. Are these symptoms the result of Macrophoma rot? Most of the research on Macrophoma rot has been on Vitis vinifera and Muscadines. Obviously, Norton is susceptible to N. ribis the pathogen that has been identified as an aggressive trunk disease pathogen. This would suggest that Macrophoma rot should be a pathogen of concern. Research in Virginia using in-vitro assays identified Captan, tetraconazole (Mettle) and thiophanate-methyl (Topsin M) to have the greatest inhibition of N. ribis mycelial growth. Further research will need to determine if these fungicides provide control of Macrophoma rot under field conditions.


Pesticide News Updates

EU Debating on Banning Mancozeb

Read the article from AgNews: Mancozeb: Facing Regulatory Challenges

Update on Mealybugs from Missouri Vineyards

In June, adult mealybugs and eggs were collected from vineyards located in Gasconade and Boone counties. The eggs were reared out and DNA was extracted. Species specific primers were used to determine which mealybug species were present in Missouri vineyards sampled. At this time only the grape mealybug Pseudococcus maritimus has been found in Missouri vineyards. The grape mealybug can acquire and transmit viruses specifically Grapevine leafroll viruses. In 2017, a survey of Missouri vineyards for viruses determined that Grapevine leafroll-associated virus 3 was found in 53% of the composite samples. With Grapevine leafroll viruses present and the grape mealybug vector in some vineyards it may be time to consider a mealybug management strategy. First, however scout your vineyards to determine if mealybugs are present.

The identification of grape mealybugs was reported to me by Jacob ConCoran, USDA. Jacob and I have been collaborating throughout the summer. Thanks to vineyard collaborators that have helped out Jacob this summer.

Cumulative Growing Degree Days (Base 50) for the Seven Grape Growing Regions of Missouri from April 1 to Aug. 29, 2020

Region Location by County Growing Degree Days1
2020 2019 30-year Average
Augusta St. Charles 2884 2981 2976
Hermann Gasconade 2770 2900 2824
Ozark Highland Phelps 2928 3051 3031
Ozark Mountain Lawrence 2918 3035 3001
Southeast Ste. Genevieve 2887 3012 3000
Boone 2916 2987 2899
Western Ray 2789 2803 2828

1 Growing degree days at base 50 from April 1 to Aug. 29, 2020. Data compiled from Useful and Useable at Click on link below to determine growing degree days in your area.

To determine the number of growing degree days accumulated in your area since April 1, use this tool.

Weather Outlook

Weather Outlook for Weekend

  • Temperatures in the mid 80s
  • Scattered thunderstorms on Monday, Sept. 7

Sept. 7 to Sept. 12

  • Below normal temperatures
  • Above normal chance of precipitationfrom Monday, Sept. 7, through Thursday, Sept. 10