Fusarium oxysporum f. sp. niveum Molecular Diagnostics Past, Present and Future

Abstract

Watermelon is an important commercial crop in the Southeastern United States and around the world. However, production is significantly limited by biotic factors including fusarium wilt caused by the hemibiotrophic fungus Fusarium oxysporum forma specialis niveum (Fon). Unfortunately, this disease has increased significantly in its presence over the last several decades as races have emerged which can overcome the available commercial resistance. Management strategies include rotation, improved crop resistance, and chemical control, but early and accurate diagnostics are required for appropriate management. Accurate diagnostics require molecular and genomic strategies due to the near identical genomic sequences of the various races. Bioassays exist for evaluating both the pathogenicity and virulence of an isolate but are limited by the time and resources required. Molecular strategies are still imperfect but greatly reduce the time to complete the diagnosis. This article presents the current state of the research surrounding races, both how races have been detected and diagnosed in the past and future prospects for improving the system of differentiation. Additionally, the available Fon genomes were analyzed using a strategy previously described in separate formae speciales avirulence gene association studies in Fusarium oxysporum races.

Keywords: Fusarium oxysporum f. sp. niveum; comparative genomics; diagnostics; distribution; effector profile; fusarium wilt of watermelon; race differentiation.

Figure 1

Fusarium wilt symptoms (A) in the field, and (B) in the vasculature (arrow).

Figure 2

Microscopic morphology of Fon (A) macroconidia, and (B) microconidia.

Figure 3

Global Fon distribution. Countries are colored based on highest race detected: green, race 1; yellow, race 2; red, race 3; and purple, race not reported. (A) Country area proportional to watermelon yield (hg/hectare), and (B) country area proportional to watermelon production (metric tons). Images produced using an online program which scales images depending on a determined variable while maintaining the original object boundaries as computed by a flow-based algorithm [27].

Figure 4

Visual differences in Fon symptoms on watermelon plants for scoring disease. (A) Score of 0, asymptomatic plants. (B) Score of 3, plants have cotyledon lesions. (C) Score of 5, plants are slightly wilted and stunted. (D) Score of 7, plants are severely wilted and stunted. (E) Score of 9, plants are dead.

Figure 5

Molecular method of Fon race differentiation adopted from Hudson et al. (2021) [24].